Patient Information

First trimester screening for Downs syndrome

The vast majority of babies are healthy, but about 1 in 100 are born with a serious mental or physical handicap. One of the commonest conditions is Downs syndrome, when the baby receives an extra chromosome 21 (Trisomy 21). It is more common with older mothers, but can occur whatever the age of the mother (see the table below).

Maternal age (yrs)

Chance of Downs syndrome

At 12 weeks

At birth

20

1 in 1070

1 in 1530

25

1 in 950

1 in 1350

30

1 in 630

1 in 900

32

1 in 460

1 in 660

34

1 in 310

1 in 450

35

1 in 250

1 in 360

36

1 in 200

1 in 280

38

1 in 120

1 in 170

40

1 in 70

1 in 100

42

1 in 40

1 in 55

44

1 in 20

1 in 30

 

Screening for Downs syndrome and diagnostic testing

The only way to be certain whether the fetus has Downs syndrome or not is by doing a diagnostic test – an amniocentesis or chorionic villus sampling (CVS). A CVS is performed between 11 and 13+6 weeks and involves taking a small amount of tissue from the placenta; an amniocentesis involves taking fluid from around the baby and is usually done after 15 weeks. Both provide a sample that contains tissue that has the same genetic make-up as the baby and will allow the baby’s chromosomes to be looked at in detail. But the problem with these invasive tests is that they can cause a miscarriage, even if the baby is entirely normal. The risk of miscarriage is about 1%.

Therefore the decision as to whether to have an invasive test or not must be taken on the basis of a non-invasive assessment of the likelihood (risk) that the baby has Downs syndrome. The most accurate way of estimating this risk is by combining the information from:

The nuchal scan

The nuchal scan is an ultrasound scan performed between 11 and 13+6 weeks of pregnancy, during which the fluid at the back of the baby’s neck (the nuchal translucency) is measured. All babies will have some fluid here, but in babies with Downs syndrome the fluid tends to be increased.

What else does the scan do?

Other things that we can do at the time of the scan are:

First trimester serum biochemistry

A blood test taken from the mother can be used to measure the levels of two hormones (free ß-hCG and PAPP-A). In pregnancies affected by Downs syndrome there is a tendency for the levels of free ß-hCG to be increased and PAPP-A to be decreased.

The combined first trimester screening test

At the end of the combined screening test you will be given a risk that the baby has Downs syndrome; it will not tell you definitely whether or not your baby has Downs syndrome, but can be used to help you to make an informed choice about whether to have an invasive test. The wish to proceed to one is a purely personal decision, but as a guide we normally offer an invasive test if the risk of Downs syndrome is 1 in 300 or worse. This of course means, that even with a high-risk result most women will still have a completely normal baby.

The 20 week scan

The best way to detect structural problems, including spina bifida, is a scan at 18 - 23 weeks and, whatever your nuchal scan shows, it is recommended that you have this scan to check the structure of the baby in more detail.

If the nuchal measurement is increased we recommend a detailed heart scan too, because a heart abnormality can be the cause of the increased nuchal fluid.

After you have delivered

Please help us by returning the outcome of pregnancy form once you have delivered your baby, so that we can monitor our service. We are always looking for ways to improve our service too and welcome any comments or feedback you may have.

Further questions

Please feel free to discuss in more detail any of the issues raised in this section at the time of your scan.

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