Screening for Chromosomal Abnormalities

Antenatal Obstetrics 

The risk of having a baby with chromosomal abnormalities (e.g Down’s Syndrome or Trisomy 21) increases with age.

With more women delaying childbearing until a later age in life, the risk of having an affected baby increases in these women. The risks are shown below e.g the risk of having a baby with Down’s Syndrome at the age of 37 is 1 in 225, the risk of having a baby with any chromosomal abnormality at this age is 1 in 122.

Having said that, most babies with Down’s Syndrome are born to younger mothers because statistically, they have more babies.

Any pregnant lady should consider screening for chromosomal abnormalities, however the first question that needs to be asked is ‘What would I do with an abnormal result?’


 There are tests which can be done to either screen for, or diagnose chromosomal abnormalities in a baby.

Screening Tests

The mainstay of screening tests now is Nuchal Screening. This is done with a blood test and ultrasound between 11 and 14 weeks. The advantage of this is that the test is non-invasive and carries no risk to the mother or baby. The disadvantage of this test is that it is not 100% accurate and false positives and negatives do occur. The test will provide you with a risk assessment – so that you can see your risk based on your age alone and then your adjusted risk after the test (which may be lower or higher depending on the result).

Some time soon, a pap smear test which picks up fetal cells excreted through the cervix, may become available. This has the potential to be an extremely accurate test with virtually no risk. Currently it is in experimental trials.

Diagnostic Tests

Diagnostic tests consist of either chorionic villus sampling (CVS) or amniocentesis. Both methods involve passing a fine needle through the abdominal wall and through the uterus (although CVS can also be done through the cervix) and placental cells in the case of CVS, or amniotic fluid in the case of amniocentesis are extracted. CVS is done earlier at around 11 weeks, whereas amniocentesis is done later at around 15 weeks.

The advantages of these tests is that they are almost 100% accurate and it is rare to have a false positive or false negative result. CVS can be done earlier which means if an abnormal result is obtained and termination of pregnancy is requested by the woman, then this can be done earlier. This is important in women who have had a previous caesarean section, as surgical termination can be done earlier. Medical termination with drugs may be the only option later in the second trimester,and this can be risky in women who have had a previous caesarean.

The disadvantage of these diagnostic tests is that they carry a risk of miscarriage, in the order of approximately 0.5 – 1%. This needs to be kept in mind when comparing with Nuchal Screening.

The decision regarding screening for chromosomal abnormalities is very individualised and will depend on every woman’s beliefs, experiences and opinions, as well as their partners. The earlier the discussion in the pregnancy, the better, as plans can be made from there.

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