Down’s syndrome is a developmental condition caused by three copies of chromosome twenty-one. It is usually called Trisomy 21, though there are two rarer forms with variations on the chromosomal cause: mosaicism and translocation. Down’s can be identified by characteristic facial features and has many associated conditions and complications that vary between cases. It happens at conception, so there is nothing the mother does or can do during pregnancy to either avoid or cause it, though conceiving when the mother is older than 35 is known to increase your risk. All women are offered screening for Down’s in pregnancy. The purpose of the screening is to decide which mothers-to-be need more invasive tests to establish whether their babies have the condition. The screening poses no risk to the health of the mother or the baby, but she can decline to have it if she wants. Many women opt to have the screening in order to make more informed decisions about their pregnancy, though screening is not always accurate. The invasive tests, which carry a small risk of miscarriage, are considered highly accurate. Many parents who learn their babies have Down’s choose to end their pregnancies.

The screening tests involve an ultrasound, during which measurements of the foetus are taken. Those measurements, along with the mother’s age and blood test results, allow for the calculation of risk. Older mothers have a higher risk naturally.

Antenatal Screening Tests for Down’s Syndrome

The Combined Test for Down’s Syndrome

The combined test is the first line and most accurate screen test for Down’s. It’s typically offered for some time between weeks 11 and 14 weeks of pregnancy and involves combining those ultrasound measurements for nuchal transclucency (the thickness on the back of the neck) with blood test results and the maternal age. Down’s is associated with a nuchal thickness greater than 3.5 millimetres, though it is not the only possible cause of this. The blood tests are for beta human chorionic gonadotropin (β-hCG), a chemical from your baby’s placental tissue, and pregnancy-associated plasma protein A (PAPP-A). A Down’s pregnancy creates unusually high levels of β-hCG and low levels of PAPA, though there are a number of other causes of both.

The Triple Test for Down’s Syndrome

The triple test is usually offered for some time between 14 and 20 weeks of pregnancy if a mother has not had the combined test for some reason. The triple test involves the maternal blood tests for the raised level of β-hCG mentioned above, a high rate of alpha-fetoprotein (AFP), and a lower than normal level of the female sex hormone oestriol.

The Quadruple Test for Down’s Syndrome

This test is identical to the triple test above but also includes blood testing for inhibin A, another hormone produced by the placenta. Elevated levels are linked to Down’s. It is another option that may be offered if it is too late for the combined test and is usually performed between 14 and 20 weeks of pregnancy.

Non-Invasive Prenatal Testing (NIPT) for Down’s Syndrome

This is a relatively new screening test. It has been available from private pregnancy care providers in the UK since 2012, but the NHS did not roll it out until 2021. It involves a simple DNA blood test from the mother. Some fragments of this DNA will be from the placental tissue and are therefore representative of her baby’s DNA. This can then be analysed for Down’s. NIPT is not as definitive a test as the diagnostic tests CVS or amniocentisis, but it does give a strong indication of whether a baby has Down’s.

Antenatal Diagnostic Testing for Down’s Syndrome

Each of the antenatal screening tests described above produces a risk score. When a baby’s risk of Down’s is greater than one in 150, which happens in around five percent of cases, the woman is offered amniocentisis or chorionus villus sampling. These tests carry a slight but real risk of miscarriage. She may also be offered or choose to have non-invasive prenatal testing, which is less definitive but carries no risk of miscarriage.

CVS involves an ultrasound-guided biopsy of your placental tissue. This is used when testing is done earlier in pregnancy, e.g., before 15 weeks. The risk of miscarriage is about 0.5 to one percent. Amniocentisis is ultrasound-guided aspiration of amniotic fluid using a needle and syringe. It is only performed later in pregnancy once there is enough amniotic fluid for testing to be safe. The miscarriage risk is lower with amniocentesis, about one percent. CVS usually gets results faster.

How does my age impact my Down’s syndrome risk, screening, and testing in pregnancy?

More Down’s babies are born to mothers under 35. This may be because this demographic has more babies generally or because women over the age of 35 are more likely to be tested and subsequently terminate Down’s pregnancy. Advanced maternal age (over 35) makes you more likely to conceive a baby with Down’s because of chromosomal abnormalities in your eggs. A woman aged 25 has a mere one in 2,500 chance of this happening, but her risk increases dramatically (to about one in 350) ten years later. Your age is most likely to impact the advice you receive during your preconception consultation and antenatal care, and you may have screening and testing more strongly recommended as a result of your age. You may also have diagnostic tests recommended to you even if your screening results do not show an increased risk, depending on your healthcare provider.

For more advice on taking care of yourself in pregnancy, labour, and birth, check out Unii’s Antenatal Classes, with prenatal video episodes taught by senior NHS midwives, an award-winning obstetrician and gynaecologist, and six other leading healthcare experts. Only at unii.com.

Antenatal Class Questions: Down's Syndrome Screening in Antenatal Care: Understanding Your Options