Screening tests during pregnancy
During your pregnancy you will be offered a range of tests and checks. Some of them will be offered to everyone; some will be offered to you because of your particular situation, or because the result of a previous test raises a question about your health or your baby's health.
There are two groups of tests carried out during pregnancy. One group of tests look at the health of the mother. The other group of tests looks at the health of the baby.
Screening tests
Most of the antenatal screening tests looking at the health of the baby do not give a definite answer one way or the other as to whether something is wrong but these screening tests are used to show which pregnancies have an increased chance (higher risk) of the baby being born with a particular condition such as Down’s syndrome. They cannot usually diagnose anything for certain, but they give information so that parents can then be offered further tests (diagnostic tests) if needed.
Most pregnancies with an increased chance of a problem turn out to develop normally. Occasionally, affected pregnancies do not show up as having an increased chance. It is also possible for unexpected problems to be picked up. For these reasons, screening tests can only be used as an indicator. They are not foolproof. You may want to ask about the chances of having a 'false positive' or 'false negative' answer where the result of the test turns out to be incorrect. Screening tests are normally blood tests or ultrasound scans.
Most people decide to have most or all of the tests, but you don't have to accept any of the tests you are offered. It is your choice. You can ask any questions you like about the tests - it's a good idea to write them down so you don't forget what you want to know.
Make sure you ask for the results of any tests afterwards. Some tests are so routine, and have been carried out for so many years, that busy staff may overlook the fact they are not an everyday experience for you, and that you may be wondering about what they are for and the meaning of the results.
No screening test is absolute, that is why it is called a screening test. For example, with blood tests everyone has variable levels of substances in their blood. It is called a range of levels. Sometimes for reasons we do not know some women have levels outside or at the very margins of these acceptable levels but their baby is normal. Sometimes babies that have something wrong, such as Down's syndrome, have levels which are inside the normal range.
Similarly, not all problems can be detected by scans, either because they are not visible (such as autism or cerebral palsy) or because the technology is as yet unable to pick them up. Sometimes problems develop later in the pregnancy after the scan has taken place.
Diagnostic tests can tell you if your baby has a particular condition. While diagnostic tests have a very high degree of accuracy and you can be almost certain the result, positive or negative, is correct you should always ask about the possibility of error.
Most diagnostic tests are offered to women who are already thought to have an increased chance (higher risk) of having a baby with a disability or chronic condition, often because of the results obtained through screening tests.
There are now national guidelines to try and make sure that all women are offered certain screening tests. Presently all NHS hospitals have to offer screening for Down's syndrome but different tests are available in different places. Some women will be offered a nuchal scan at 10-13 weeks in other places women are offered what is thought to give the most accurate test, a nuchal scan combined with a blood test. In some places women of a certain age (usually over 35) are offered a diagnostic test of an amniocentesis or CVS instead of screening (but they can opt for screening if they prefer). These diagnostic tests give an almost certain answer as to whether the baby has Down’s but the test itself has a small risk of miscarriage so women may want to go for the screening test first. Overall about one in 30 (3%) of women screened have an increased chance (higher risk) and are offered a diagnostic test.
How accurate are scans in telling that something is wrong?
Scans are not foolproof and it is not possible to pick up all problems including such problems as autism or cerebral palsy but they will usually pick up obvious structural things that are not developing properly in the baby.
The table below is taken from the leaflet ‘Screening tests for you and your baby’ produced by the National Screening Committee’ it shows the 11 problems that the NHS Fetal Anomaly Screening Programme are audited against. Your sonographer will be looking specifically for these problems during the scan. Some problems are quite common, others are very rare and they will be explained to you.
|
Problem
|
What the problem is
|
Chance of being seen
|
|
Anencephaly
|
This is abnormal development of the brain and the bones of the skull. Sadly, babies with this problem cannot live once they are born and die soon after birth.
|
98%
|
|
Open spina bifida
|
Spina bifida is when the baby’s spinal cord has not developed properly and there is a gap or split in the spine.
|
90%
|
|
Cleft lip
|
This happens when certain parts of the baby’s face and particularly the lips do not join together properly.
|
75%
|
|
Diaphragmatic hernia
|
This occurs when the baby’s diaphragm does not fully form. The diaphragm is a muscle that helps us breathe and it keeps the heart and lungs separate from the bowel and the rest of the organs in the abdomen.
|
60%
|
|
Gastroschisis
|
This is a defect or ‘hole’ in the baby’s abdominal wall to one side of the umbilical cord (usually the right side). Some of the bowel escapes through this hole and develops outside of the baby’s abdomen.
|
98%
|
|
Exomphalos
|
Exomphalos occurs when the abdomen fails to close around the base of the umbilical cord during the early development of the baby. Allowing the contents of the abdomen to enter the umbilical cord.
|
80%
|
|
Serious cardiac abnormalities
|
These include a range of heart abnormalities which mean the baby will need medical attention very soon after birth.
|
50%
|
|
Bilateral renal agenesis
|
This means that both the baby’s kidneys have not developed. Sadly, babies with this condition die shortly after birth as they cannot live without kidneys.
|
84%
|
|
Lethal skeletal dysplasia
|
Lethal skeletal dysplasia is a problem which affects the size and shape of arms, legs, the body or sometimes the skull. The chest and lungs of these babies do not fully develop, which means that they do not survive.
|
60%
|
|
Edwards’ syndrome (Trisomy 18)
|
This rare chromosome disorder occurs when a baby has three copies of chromosome 18 instead of the usual two.
|
95%
|
|
Patau’s syndrome (Trisomy 13)
|
Trisomy 13 is a chromosomal disorder. It occurs when a baby has three copies of chromosome 13 rather than the usual complement of two.
|
95%
|
Types of test during pregnancy
Screening of the mother's health and screening of the baby's health often get confused. This site is mainly about screening the baby, but we also include some information here about tests for the mother.
Blood tests
A number of different blood tests, done on small samples of blood only, are routinely offered at the beginning of pregnancy. Blood tests are carried out for many reasons:
- • to check the mother's blood group.
- • to monitor the mother's health (for example checking iron levels for anaemia), and look for conditions which may develop in pregnancy such as diabetes and pre-eclampsia.
- • to screen her for conditions she may already have which can affect her or the baby (such as HIV or various sexually transmitted diseases)
- • to check whether the parents are carriers of inherited conditions such as sickle cell anaemia (this is provided routinely in some areas, whilst in other areas it is targeted only at 'at risk' groups)
- • to screen for immunity to infectious conditions such as rubella which can harm the baby if the mother catches them while pregnant
- • to screen for abnormalities in the baby e.g. spina bifida, Down's syndrome
(For more information see information section on 'Types of test - blood tests')
Urine tests
You will be asked to produce a sample of urine at your antenatal appointments to check for any infection and to monitor the presence of protein which can be one of the symptoms of pre-eclampsia. An early check on your urine may be made to check for the sexually transmitted disease chlamydia. This can be treated safely and effectively with antibiotics.
Weight and height check, done at the start of pregnancy, as significant over or underweight can indicate a risk of problems later on. In most cases, you won't be measured again
Blood pressure, probably carried out at all your antenatal appointments as a rise in blood pressure points to possible problems with your health and the well-being of your baby such as pre-eclampsia.
Size and position of the baby
In addition, the midwife will probably check the height of your uterus at most antenatal appointments by feeling your abdomen and possibly measuring with a tape. This helps assess the growth of the baby. She will be looking for the position of the baby as well, to see where the head is. She is also likely to listen to the fetal heart with a special ear trumpet placed on your abdomen, a stethoscope, or a hand-held electronic device called a Sonicaid.
The midwife should always explain what she is about to do, and let you know what she finds.
Dating scan
An ultrasound scan to help confirm or work out the length of your pregnancy so far and the likely date around which your baby will be born. Sometimes the dating scan can also detect abnormalities.
(For more information see information section on 'Dating scans')
Nuchal scan
Using ultrasound, the back of your baby's neck is carefully observed and measured.
(For more information see information section on 'Nuchal scan')
18-20 week anomaly scan
An ultrasound scan that observes and measures the growth of your baby and some of his or her major organs to check for problems.
(For more information see information section on '18-20 week anomaly scan')
Last reviewed September 2010.
Last updated September 2010.
The conditions that screening and diagnosis are looking for
Chromosomal disorders
Every cell in the body normally has 23 pairs of chromosomes, formed of molecules of DNA and containing the body's genetic information. One of the 23 pairs of chromosomes is the sex chromosomes, defining the body as male or female. Babies get one half of their chromosomes from their mothers and the other half from their fathers.
Down's syndrome
Is the most common chromosomal disorder, sometimes called trisomy 21, because it is caused by the existence of an extra chromosome 21, making three instead of a pair. Usually Down's syndrome does not run in families except in a small percentage of cases where one parent carries a chromosome rearrangement.
Babies born with Down's syndrome have some characteristic physical features. They have widely varying degrees of learning disabilities and sometimes additional physical health problems. Babies with Down's syndrome have a high risk of having heart and/or bowel disorders which may need surgery. Although scans may be able to tell you if a baby with Down's syndrome has additional physical complications, it can be difficult to be sure how severe these are. It is impossible to tell before birth what level of learning disability the baby will have.
In 2008/9 there were 1844 diagnoses of Down syndrome, 64% of which were detected before birth and 92% of affected pregnancies were terminated and 8% were continued. Some of the continued pregnancies miscarry naturally, some end as still births, and approximately 6% of those prenatally diagnoses are live births. In 2008/9 there were an estimated 687 Down’s syndrome live births (the figures quoted are from
The National Down Syndrome Cytogenetic Register for England and Wales: 2008/9 Annual Report).
The older a mother is the more chance she has of having a baby with the condition. For example, the chance of having a baby with Down’s syndrome is 1 in 1500 for women who are 20 years old, 1 in 900 for women who are 30 years old, and 1 in 100 for women who are 40 years old.
What does it mean to be told you're at high risk for Down's? How can I make sense of the figures?
Usually you will be told you have an increased chance (higher risk) if the test indicates your chances of having a baby with Down's syndrome are higher than 1 in 200.
For example, a 30-year old woman gets a result which shows her risk of having a baby with Down's syndrome, is about 1 in 500. This means that if there were 500 people in exactly her situation, 1 woman would have an affected baby and the other 499 would not. There is a chance that she is the one, and we cannot know that without further tests, but she is much more likely not to have an affected baby. If the result were 1 in 200, this would be considered an increased chance (higher risk) and she would be offered further diagnostic tests.
Sometimes you will hear risk presented as a percentage. The following table shows two different ways of presenting the same information.
|
Chance of
Down's syndrome
|
Chance of an
unaffected pregnancy
|
|
1 in 4
|
25%
|
3 in 4
|
75%
|
|
1 in 5
|
20%
|
4 in 5
|
80%
|
|
1 in 10
|
10%
|
9 in 10
|
90%
|
|
1 in 20
|
5%
|
19 in 20
|
95%
|
|
1 in 30
|
3%
|
29 in 30
|
97%
|
|
1 in 50
|
2%
|
49 in 50
|
98%
|
|
1 in 100
|
1%
|
99 in 100
|
99%
|
|
1 in 200
|
0.5%
|
199 in 200
|
99.5%
|
|
1 in 500
|
0.2%
|
499 in 500
|
99.8%
|
|
1 in 1000
|
0.1%
|
999 in 1000
|
99.9%
|
Patau's Syndrome (trisomy 13) and Edwards' syndrome (trisomy 18) are much rarer than Down's syndrome. They are both very serious, life-limiting conditions and affect many aspects of growth, development and learning.
Of those born alive, 90% of babies born with Edwards' syndrome die within their first year of life. Most babies with these abnormalities do not survive pregnancy and are miscarried. 90% of babies born with Patau's syndrome die within 3 months.
Turner's syndrome affects only females. Most babies with Turner's syndrome miscarry. Those that are born do well but tend to be short and are usually infertile. The short stature can be treated with growth hormone. There is also an increased risk of having a correctable heart defect. Turner's occurs in about 1 in 2000 female births. There are varying levels of severity of Turner's syndrome and children with Turner's syndrome may have specific learning difficulties, but usually have a normal IQ. For support and information:
Turner Syndrome Support Society.
Klinefelter's syndrome affects only males. It is caused by an extra x chromosome in males and occurs in 1 in 650 live boy births. It is often not diagnosed until the man is undergoing fertility investigations, because it causes infertility. Men with it tend to be tall and IQ is usually within the normal range, but often about 10 points lower than their siblings. It can be associated with some behaviour and learning difficulties.
For support and information: Klinefelter's Syndrome Association. There are a number of other chromosomal disorders. If you are having an amniocentesis or CVS, ask what disorders are being checked for.
Triple X only affects females. Girls and women with triple Xhave an extra X chromosome. It is believed that Triple X occurs in about 1 in 1,000 girl births but it is often never diagnosed. Baby girls with Triple X are known to have some mild physical and speech developmental delays and may need some extra learning support. They experience rapid growth at 4-13 years, with especially long legs and can have difficulties in making friends at school age, which normalises in adolescence. They can have an increased vulnerability to behavioural and social stress.
XYY only affects males. It is caused by an extra Y chromosome and is believed to occur in 1 in 1000 boy births. It is often not diagnosed. Boys are of normal appearance but often tend to be tall. There are no increases in medical problems or illnesses and their Intellectual ability is usually in the normal range, but they can have an increased need for learning support. They can also have an increased vulnerability to behavioural and social stress. As children they can often have tantrums. The components of social difficulties are complex, but include difficulties with turn taking and negotiating, ignoring social cues, inability to see other people’s points of view, tactlessness and anxiety in groups.
For more information on Triple X, XYY and other rarer chromosomal abnormalities see the
UNIQUE website.
Congenital heart defects: occurs in around 7 of every 1,000 pregnancies. Half of these babies will probably need some form of medical treatment or surgery. There are several different forms of heart defect. Some defects run in families, but most defects have no obvious known cause. Some may be caused by the mother's health during pregnancy, or by drugs or medication she has used.
Neural tube defects: if the neural tube, which forms the brain and spinal cord, fails to close during the first weeks of fetal development, a portion of the spinal cord and nerves may fail to develop. This results in spina bifida. Other consequences of neural tube defect include anencephaly (where the brain and skull fail to develop); hydrocephalus (where there is a build up of cerebro-spinal fluid in the skull). Spina bifida and anencephaly account for up to 95% of all neural tube defects in equal proportions.
There is no definite cause for neural tube defects, though there is a hereditary factor, and supplementing the diet with folic acid around the time of conception and for the first three months has been shown to reduce the incidence. The number of Neural tube defects has declined significantly in the last 30 years and now occurs in approximately 0.8/1000 total births. Most are detected before the birth.
Some epilepsy medication increases the risk for spina bifida.
Sickle cell disease and thalassaemia: these conditions affect the red blood cells. Your baby is at risk if you are from a family with an inherited trait, which is present in some ethnic groups. Both partners need to be carriers to have an affected baby.
In the UK, sickle cell is most common in people of Asian, African, Caribbean, Mediterranean or Middle Eastern decent. About 1 baby in 2000 is born with sickle cell disease in the UK
(Patient UK 2008b). Currently there are 12,500 patients with sickle cell in the UK at present (All-Party Parliamentary Group on Sickle Cell and Thalassaemia – ‘
Sickle Cell Disease and Thalassaemia: A Health Check’ July 2009)
Thalassaemia is most common in people from Cyprus and some parts of Asia. There are around 1000 beta thalassaemia major patients in the UK, with more than 214,000 people in the UK carrying the beta thalassaemia gene. (All-Party Parliamentary Group on Sickle Cell and Thalassaemia – ‘
Sickle Cell Disease and Thalassaemia: A Health Check’ July 2009 )
Other conditions
There are many other conditions which may be detected antenatally, such as cleft lip and palate, and gut problems and if you have a particular concern, share it with your health professional. Not all conditions such as cerebral palsy and autism show up with testing, however, and not all testing is entirely accurate. Again, this is something for you to discuss at the time, so you are aware of what can and can't be done.
Last reviewed September 2010.
Last updated September 2010.
Types of test
Blood tests
The purpose of blood tests in pregnancy was a common source of confusion for women. Blood tests are carried out for many reasons:
- • to check the mother's blood group, including whether they are Rhesus positive or negative (for more information see below).
- • to monitor the mother's health (for example checking iron levels for anaemia), and look for conditions which may develop in pregnancy such as diabetes and pre-eclampsia
- • to screen her for conditions she may already have which can affect her or the baby (such as HIV or various sexually transmitted diseases such as syphilis or hepatitis).
- • to check whether the parents are carriers of inherited conditions such as sickle cell disease and thalassaemia In some areas where sickle cell anaemia and thalassaemia are prevalent amongst the population, all pregnant women may be tested for these condition to check if they are carriers. Sometimes screening is offered only to those women from an ethnic background with a higher risk.
- • to screen for immunity to infectious conditions such as rubella which can harm the baby if the mother catches them while pregnant
- • to screen the baby for conditions such as Down's syndrome or spina bifida.
A Quick Guide to Blood Screening Tests
|
Looks for
|
When
|
Results
|
|
Mother's blood group - ABO and Rhesus type
|
Early Pregnancy
28 weeks
|
Within 2 weeks
|
|
Mother's haemoglobin
(iron)count - anaemia
|
Regular intervals
|
Within 2 weeks
|
|
Immunity to German Measles in mother
|
Early pregnancy
|
Within 2 weeks
|
|
Syphilis infection
of mother
|
Early pregnancy
|
Within 2 weeks
|
|
Hepatitis B infection
of mother
|
Early pregnancy
|
Within 2 weeks
|
|
HIV, the virus that can cause AIDS and affect the baby
|
Early pregnancy
|
Within 2 weeks
|
|
Chances of Down's syndrome and other chromosomal problems, e.g. Edwards' syndrome
|
15-18 weeks
|
Within 2 weeks
|
|
Carriers of sickle cell and thalassaemia
|
Early pregnancy
|
Within 2 weeks
|
|
Alpha-fetoprotein (AFP) - increased risk of Spina Bifida and other neural tube defects
|
15 -18 weeks
|
Within 2 weeks
|
Some centres are also using blood tests to look for carriers of the inherited condition cystic fibrosis.
Your first blood test uses the same sample of blood to check for:
- • Your blood group,
- • Your 'blood count' checking levels for anaemia,
- • Evidence of various illnesses and infections, including rubella (German measles), syphilis, hepatitis.
- • HIV/AIDS
- • Carriers of sickle cell and thalassaemia
All pregnant women are likely to be offered further blood tests at least once or twice in later pregnancy.
Blood tests to check the baby
A blood test may be carried out to assess the risk of Down's syndrome and other conditions.
The blood sample is tested for biochemical markers. These markers are substances in the mother's blood which tend to be different if the baby has Down's syndrome or some other chromosomal abnormality. There are a number of different investigations carried out by different units. Ask the midwife or doctor the name of the test, its accuracy, and its rate of false positives. Approved tests in England and Scotland must have a minimum 75% accuracy rate (that is, they detect 75% of babies with Down's syndrome) and less than 3% false positives. In Wales the minimum accuracy rate is 60% and less than 5 per cent false positives. (UK National Screening Committee)
There are 3 main groups of tests available for Down's syndrome screening:
- • The nuchal scan test with or without a blood test for biochemical markers which is carried out at between 10 and 14 weeks. Different places check for different combinations of biochemical markers.
- • The triple test looks for 3 biochemical markers and the quadruple test for 4 biochemical markers. These are both done at 15 to 20 weeks. For this test to be accurate there should have been a dating scan carried out before the blood is taken. This test will look for the risk of Down's syndrome and Spina Bifida.
- • There are also tests which combine nuchal scans and blood tests at 10 to 14 weeks plus a further blood test at 15 to 16 weeks. This test is called the integrated test which it claims has the highest detection for Down's syndrome. This test also checks the baby for Spina Bifida.
If you have had a result from a nuchal scan, it is advisable not to have the triple test as well, because it will also give you a risk for Down's syndrome and having two different assessments can be confusing. However, you may have a blood test at 15-18 weeks of pregnancy, sometimes called an AFP test, which looks for spina bifida, indicated by high levels of a substance called alpha-fetoprotein (AFP) in the blood. The test picks up 9 out of every 10 babies with spina bifida.
However, high levels can also indicate twins, an incorrectly dated pregnancy, or nothing significant at all. In most cases, the scan you are then offered to check the baby's spine shows that everything is fine.
You can get more information about these tests and their performance rates from the Royal College of Obstetricians and Gynaecologists guidance on antenatal care
www.rcog.org.uk.
Rhesus factor
This is a protein on the surface of every red blood cell. Most people (85 to 95 per cent) are Rhesus positive. The Rhesus factor only matters if you are Rhesus negative and your baby's father is Rhesus positive, and if your baby has inherited the positive factor from him. In this case, your own blood could 'fight' the 'foreign' blood of your baby - antibodies may cross the placenta and attack the red blood cells of your baby. This is more likely to be an issue in a second or subsequent pregnancy. It's dangerous for the baby, who could be born with a serious blood disorder.
The situation is monitored by regular blood checks, and you may be given Anti-D immunoglobulin injections to counter the effect as a preventative measure.
Scans
Is there any evidence that ultrasound can be harmful to my unborn baby?
This has been a debate for many years but to date there has been no overwhelming conclusive evidence that ultrasound can harm your baby if it is used in the correct manner. It appears to be safe but it remains sensible not to do more scans than necessary. All sonographic departments have guidelines to ensure the equipment they use is safe.
A quick guide to ultrasound scans (with or without blood tests)
|
Type of scan
|
Looks for
|
When
|
Results
|
|
Nuchal translucency scan (NT) (with or without blood test)
|
Chances of Down's syndrome
|
10-14 weeks
|
Immediate or later with blood test results
|
|
Dating scan
|
Baby's size, heartbeat, number of babies
|
Early pregnancy
|
Immediate
|
|
Detailed or anomaly scan
|
Certain physical problems, such as neural tube defects, clues for Down's syndrome, heart and kidney problems, other limb or organ abnormalities
|
18-20 weeks
|
Immediate
|
Sometimes people doing the scan are happy to talk as they do it, others prefer to talk about it once they have finished doing the scan. Be prepared for staff to be silent and concentrating, but it doesn't necessarily mean they have spotted something wrong - you may want to ask them first whether they will talk to you during it or not. Always ask to see the scan if you have not been shown it and want to see it.
Nuchal scan
This is a specialised ultrasound scan of the back of the neck, done at some time between 10 and 13 weeks 6 days. In babies with Down's syndrome and some other chromosomal conditions, there is a tendency for more fluid to collect in this part of the neck; the sonographer [the person operating the scanning equipment] can measure the amount of fluid and assess if it is within normal limits. The test is sometimes called a nuchal translucency test, or a NT test. It is increasingly common that it is offered, and sometimes as part of the dating scan.
The scan has to be done at the right time, because accurate measurements are not possible before 10 weeks and at 14 weeks and after, the extra fluid is absorbed by the fetus. This is a good reason for making sure you get in touch with your GP or midwife within the first few weeks of pregnancy, so there is time to consider and arrange a nuchal scan. The risk of Down's syndrome is calculated by factoring in your age to the result - older mothers (over the age of 35 at the time of birth) already have an increased chance (higher risk) of having a baby with Down's syndrome.
Understanding screening test results is not always straightforward, because most give only a risk or probability that something is wrong with the baby. Nuchal scan results are usually presented as a probability (for example a 1 in 100 chance of something being wrong) or sometimes a percentage (a 1% risk). These statistics can also be presented the other way round, as a 99 in 100 or 99% chance that the baby will be fine. (For a table showing ways of presenting risk see - The conditions that screening and diagnosis are looking for).
If the nuchal translucency measurement is large and the chromosomes are normal, then there is an increased chance (higher risk) of a heart defect. This would then be followed up with a more detailed scan of the baby's heart, later in the pregnancy called an echo. (For more information see 'Further scans to aid diagnosis, including scans of the baby's heart).
Dating scan
This is most often offered to you at some time between about 10 and 13 weeks of pregnancy. And is now sometimes done at the same time as the nuchal scan.
The sonographer (the person operating the scanning equipment) will firstly check for twins or more. Then he or she will take some measurements, for example, the length of the baby's body, and the diameter of the skull, the length of the thighbone. These all give an accurate assessment of the length of your pregnancy. From this, your 'due date' can be calculated. Occasionally some suspicion of abnormalities may also show up at this stage, including the possibility of a severe chromosomal condition which will need to be confirmed by other tests.
Ultrasound scans performed at this early stage in pregnancy are fairly accurate for dating purposes. Most studies show that for the majority of pregnancies, they are accurate to within just a few days. They are more accurate than working out the date from the first day of your last period, even when you are sure when that was.
Most hospitals will offer pictures sometimes for a small charge or donation. In some instances even when there is a problem discovered a picture will be offered as it is thought to be helpful to have a keepsake of your baby if you decide to have a termination. If it is not offered please ask the staff for one.
18-20 week fetal anomaly scan
This should now be routinely offered to you in all maternity units. The aim is to assess your baby for major structural abnormalities - this means checking his or her major organs (including the heart), limbs and skeleton. The growth of your baby is also assessed, and your likely due date confirmed.
You should ask your midwife, the sonographer [person operating the scanning equipment] or your consultant precisely which problems they are looking for. Some units will give you a written leaflet to help explain it. Not all scans examine exactly the same things, or they may differ in how 'in depth' the assessment is. For example, details of the way the baby's heart functions may be assessed in some units, and not in others, depending on the skills and equipment available.
You need to have a chance to think about, and possibly discuss with your professional carers, the implications to you if the scan reveals a problem. The overall detection rates for ultrasound screening are 83% for abnormalities incompatible with life, 50%for serious abnormalities where survival is possible and 16%for those requiring immediate care after birth (statistics from the Royal College of Obstetricians and Gynaecologists
report ‘Termination of Pregnancy for Fetal Abnormality in England, Scotland and Wales’ May 2010) but within this figure, some problems are more likely to be revealed than others.
For example, 90% of cases of spina bifida (where the spine fails to develop completely) show up on this scan. But only 50% of major heart defects are picked up. You can get more information about different conditions in the information section on 'The conditions that screening and diagnosis are looking for?' and on the Healthtalkonline congenital heart disease website
www.healthtalkonline.org/heart_disease/Congenital_Heart_Disease
Some observations, such as heart or bowel problems, may indicate the presence of an underlying problem, such as Down's syndrome.
Depending on the nature of the problem, you may want to consider a termination, if this happens to you; alternatively, you may want to think about it as being the chance to be prepared for the birth of a baby with a severe, life-threatening or life-limiting condition.
Note: ultrasound scans are usually carried out by moving a hand-held sensor over your abdomen. The information picked up in this way is then transmitted to a screen, which you and the sonographer can see. You need to have a bare abdomen for the probe to make contact with your skin, but you can keep your underwear on.
Sometimes, in early pregnancy, or if the baby is in an awkward position, or if your cervix needs to be examined, you may have a transvaginal or an internal scan. This means the probe is inserted into your vagina, so you will need to remove your underwear.
Ultrasound scans are not painful, but they can sometimes be a bit uncomfortable, especially because some ultrasound scans need you to have a full bladder to be effective.
Diagnostic tests (CVS & amniocentesis)
A quick guide to diagnostic tests
|
Type of test
|
Looks for
|
When
|
Results
|
|
Chorionic Villus Sampling (CVS) - takes a sample of placenta
|
Definite result for Down's syndrome and other chromosome problems
|
From 10 weeks
|
Full results take from 2 - 3 weeks. Some places offer a quick (limited) result in 2 - 3 days.
|
|
Amniocentesis - takes a sample of fluid from around the baby
|
Definite result for Down's syndrome and other chromosome problems
|
From 15 weeks
|
Full results take from 2 - 3 weeks. Some places offer a quick (limited) result in 2 - 3 days.
|
|
More detailed scans
|
Further assessment of physical problems, such as neural tube defects, heart and kidney problems, other limb or organ abnormalities
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As a follow-up to other scans - see above for timing
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Immediate
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Diagnostic tests are different from screening tests and are usually offered once screening tests have been carried out to see if the baby has a particular condition, if the screening test result has reported an increased chance (higher risk). These diagnostic tests are to find out for sure, but often have some risk attached to the test itself.
CVS stands for chorionic villus sampling, which is carried after 10 weeks of pregnancy. If the test if carried out earlier, there may be a risk of damaging the baby. The test removes a small sample from the tissues that go on to form the placenta. The sample is then tested for chromosomal disorders e.g. Down's syndrome, or in special cases some inherited conditions.
It's offered to women known to have an increased chance (higher risk) of having a baby with one of these disorders. Risk of miscarriage caused by CVS itself is about 1%-2%.
It is an accurate test, and false results are rare. However, you may be asked to return for a repeat test, or to opt for amniocentesis, which gives fewer ambiguous results, because the sampling was not successful or the results are not clear.
The test is usually carried out by passing a thin needle through the abdomen (under a local anaesthetic). The surgeon guides the instrument by watching an ultrasound screen which views inside the uterus. A few cells are removed, and results usually take 10 to 14 days (ask, however, as different units have different results times). Because the cells of the chorionic villi have the same DNA as the embryo, any abnormality in the embryo will show up in the sample when analysed in the laboratory (some centres offer quick results in 1-3 working days, using the FISH and PCR tests, see CVS/Amniocentesis and the FISH and PCR tests for more details).
Some women find the procedure does not hurt, although they may find it uncomfortable. Other women may experience some pain.
Amniocentesis
Amniocentesis, or an 'amnio', is offered from 15 - 16 weeks of pregnancy. The test removes a small amount of the amniotic fluid surrounding the baby in the uterus. The fluid is then examined in the laboratory. The test looks for chromosome abnormalities, for example for Down's syndrome and other chromosomal and inherited disorders, as well as for neural tube defects such as spina bifida and anencephaly.
Amniocentesis is offered if you and your doctor or midwife decide you have an increased chance (higher risk) of having a baby with an abnormality. It has a risk of miscarriage, caused by the amniocentesis itself, of about 1 %.
It is very accurate, but occasionally the sampling is unclear and you may be asked to come in once more for a repeat test.
The doctor inserts a needle through your abdomen, and using ultrasound helps it go to the right place (avoiding the baby and the placenta). Some centres will do this under local anaesthetic while others will not. A sample of amniotic fluid is collected using a syringe. The lab can then isolate skin cells from the baby and examine them for Down's syndrome. Results usually take 2-3 weeks, but limited results may be available earlier. Even though you may be having a test for Down's syndrome, the CVS or amniocentesis may by chance turn up another chromosome abnormality either more serious, or less serious.
Some women find the procedure does not hurt, although they may find it uncomfortable, whilst others may experience some pain.
Rarely, a test called cordocentesis (fetal blood sampling) may be carried out later in pregnancy (after 18 weeks). A sample is taken from the umbilical cord, and can provide fast results, but has a miscarriage risk of 1-2%
CVS and amniocentesis can also test for specific genetic conditions, such as sickle cell anaemia and thalassaemia, cystic fibrosis, etc. and are particularly offered for those families known to be at risk, where both parents are known to be carriers. The conditions can also be tested with fetal blood sampling, although this is now rare.
You can find out more by going to the UK Thalassaemia Society
www.ukts.org and the Sickle Cell Society
www.sicklecellsociety.org. These tests are also used to test for cystic fibrosis, if you and your partner are both known to be carriers of this condition. For more information, contact the Cystic Fibrosis Trust
www.cftrust.org.uk.
CVS/Amniocentesis and the FISH and PCR tests
It usually takes 10-20 days to get the full results from these tests. However, some centres offer quick results in 1-3 working days, using the FISH and PCR tests (Amnio-PCR and Amnio + FISH are molecular tests which provide a rapid diagnosis of the three most common chromosome abnormalities including Down's syndrome). It is important to remember that even though you may be reassured that the baby has not got Down's syndrome by the FISH and PCR result, very occasionally another chromosomal condition is found later following full examination of all the chromosomes.
Further scans to aid diagnosis, including scans of the baby's heart
Fetal cardiac scans
Detailed ultrasound scanning of the baby's heart, called a fetal cardiac scan, or a fetal echo (short for fetal echocardiogram), can take place at any point in pregnancy between 10 and 14 weeks. However, the skill and knowledge, and the equipment, to offer a cardiac scan as early as this is not yet available everywhere.
More often, the heart is checked with a fetal echo at about 20 weeks, offered to you if an earlier test or scan has shown your baby may have a heart defect, or Down's syndrome, or if there is a history of fetal heart abnormalities in your family. You may have to visit a specialist centre for this to take place, and you may need a series of echoes.
The fetal echo aims to detect abnormalities in the structure of the heart, disturbances in the rhythm of the heartbeat (arrhythmia), or defects in the way the heart functions.
Without specialist heart scanning, only about 50% of serious fetal heart defects are spotted. With the best and most up to date scanning, this percentage rises. If you are offered a fetal echo, ask about its accuracy, ask about rates of false positives and negatives in your unit, and about the implications and possibility of treatment, before or after the birth. At present, some treatment can be offered for arrhythmia while you are still pregnant; and surgery to correct some other abnormalities needs to be done after the birth.
If a heart defect is detected, it may also affect the choice of birth method, (caesarean section or vaginal delivery).
You may consider the option of terminating the pregnancy if your baby has a severe heart problem.
Other scans
If the 18-20 week fetal anomaly scan shows some physical problems with the baby. You may then be offered further more detailed scans to assess these problems such as nuchal tube defect, kidney problems and other limb or organ abnormalities.
Last reviewed September 2010.
Last updated September 2010.
Your choices
Can I choose whether or not to have certain tests?
You can choose which screening tests you want or do not want. Most people decide to have most or all of the tests, but you don't have to accept any of the tests you are offered. It is your choice. Make sure you understand what all the tests involve and why they are being done before making a decision.
I know I would never consider ending a pregnancy if there was something wrong with the baby. Should I go for screening?
It depends on your personal choice. Some people find it helpful to know in advance if there is something wrong with the baby and be able to prepare, or perhaps give birth in a hospital where specialist help/surgery will be available quickly.
Staff are not allowed to deny you screening simply because you are not sure you would ever end the pregnancy.
Different people have differing needs for certainty and information, and whilst for some not knowing is preferable, for others the uncertainty is intolerable. Also some people change their mind when they actually get bad news and even though they did not think they would ever have a termination may decide to do so. In deciding about having the test you have to weigh up how it will feel going through pregnancy knowing something might be wrong. As the screening tests for nearly all conditions will not give a definite answer, if the screening test comes back with an increased chance (higher risk), you will then need to decide whether to go on and have a diagnostic test e.g. amniocentesis or CVS, both of which have a small risk of miscarriage of what may be a healthy baby.
Will I be able to find out how severe my baby's condition is?
This will depend on the condition. An ultrasound scan can only give an indication of structural abnormalities. It will NOT be able to give you a complete picture of the problem and will not be able to tell you about intellectual ability. Therefore if your baby is diagnosed with Down's syndrome it will not be possible to know how serious learning problems will be in the future though some of the heart problems that can be associated with Down’s syndrome will be picked up on a scan.
It may be possible to predict the severity of any heart problems or problems with the spine and spinal cord from a scan but even then predicting the full story and severity may not be possible. For some severe chromosomal conditions, sometimes it is known that the condition is always severe and that the baby is unlikely to survive, but ask to speak to a clinician that specialises in that condition before making any decisions.
How quickly will I have to make up my mind whether to go ahead with the pregnancy if something is found to be wrong?
If an abnormality is found during the pregnancy you may need time to decide what you want to do. You should not feel rushed into making a decision. Many women feel that they would prefer to have an early termination and this is possible. Later terminations do involve giving birth. Termination for a very severe abnormality can happen at any time.
Some women decide to continue the pregnancy and allow the baby to die naturally. Many others would rather not face the rest of pregnancy knowing their baby will die anyway, and ask for labour to be induced. Either way you can still see the baby after birth and spend time with him or her, take photographs etc.
Around 700,000 women get pregnant in the UK every year. Over 95% of these pregnancies result in the birth of a healthy baby (UK Screening Portal www.screening.nhs.uk/ANNB). For 35,000 of these, antenatal testing reveals the risk of a serious abnormality.
In 2008, about 1950 terminations (out of a total of 195,000) were carried out because of a fetal abnormality. About 124 of these were classified as late terminations, carried out after the 24th week of pregnancy, which is permitted in the UK only when the mother's health is in danger or when 'severe abnormality' has been detected. (Statistics from the Royal College of Obstetricians and Gynaecologists
report ‘Termination of Pregnancy for Fetal Abnormality in England, Scotland and Wales’ May 2010).
Whether or not you wish to go ahead with the pregnancy you may need the chance to discuss it. For general information, you may find the following organisation helpful:
