Pregnancy blood tests – Down syndrome screening

Down syndrome (also called Trisomy 21) is a genetic condition that can happen by chance, and affects around 1 in 700 pregnancies. Read more about Down syndrome. There are two much rarer genetic conditions (Edwards and Patau syndromes) that are also screened for in these tests. Since they are rarer, the tests are often known simply as Down syndrome tests.

Babies affected by one of these genetic conditions have an increased risk of miscarriage, stillbirth, death as a baby, or a wide range of long-term physical and mental disabilities.


Image credit: canva

Screening tests

MSS1 (maternal serum screening first trimester) or ‘combined screening test’

This is performed between 11 and 14 weeks, and involves having a free blood test and an ultrasound ‘nuchal translucency (NT)’ scan which you may have to pay for. This is the commonest test currently performed in Aotearoa New Zealand. 

MSS2 (maternal serum screening second trimester)

This is performed between 14 and 20 weeks, and involves a single free blood test. This is the least accurate test, and is usually only suggested if you book in for pregnancy care after 14 weeks of pregnancy. You are also recommended to have an ultrasound scan (which you may be asked to pay for) to confirm pregnancy dates, count the number of babies and check for any early major problems, although this is not part of the MSS2. 

NIPT (Non-invasive prenatal testing)

This is a newer test, which is commonly used in many other countries. It involves a single blood test, which can be taken any time from 10 weeks onwards and looks for genetic material from your baby in your bloodstream. It is much more accurate than the other 2 tests, but more expensive for you. Read more about non-invasive prenatal testing (NIPT). 

An ultrasound scan is also recommended around 12 weeks to make sense of the results.

These tests all have advantages and disadvantages, and you should consider what test is best for you, and preferably discuss it with your health professional and your family/whānau.

No screening tests are 100% accurate – this means that a ‘high risk’ test result does not mean your baby is definitely affected by a genetic condition, while a ‘low risk’ result is not a guarantee that the baby is unaffected.

If you receive a ‘low-risk’ result from your test, you will continue with routine pregnancy care, including a detailed ultrasound at 18–20 weeks, which will occasionally detect problems with how a baby is developing.

If you receive a ‘high risk’ result from your test, you will be referred to a specialist to discuss the options, which include no further testing. You will have a detailed ultrasound scan and be offered either NIPT (which you will need to pay for) or an ‘invasive’ needle test to be 100% sure whether or not your baby is affected. Invasive tests (called chorionic villous sampling or amniocentesis) carry a small risk of miscarriage.