One of the first decisions for many couples is whether to have Down syndrome screening in pregnancy. For those who choose to test, there are 2 main options – non-invasive prenatal testing (NIPT) and the nuchal translucency. Although people usually think the screening is just for Down syndrome, we also screen for some other common chromosomal problems. These tests happen between weeks 10-14.
What are chromosomal problems?
Our DNA (genetic information) is organised in long strands called chromosomes. Normally, a person has 46 chromosomes, organised into 23 pairs. Sometimes, a baby develops with an abnormal number of chromosomes. This leads to a variety of developmental problems. The most common chromosomal problems are:
- Down syndrome (Trisomy 21): caused by having an extra copy of Chromosome 21. The risk of Down syndrome increases with the age of the mother. People with Down syndrome have a characteristic appearance, particular health problems and some intellectual disability.
- Edwards syndrome (Trisomy 18) and Patau syndrome (Trisomy 13): rarer than Down syndrome but more severe. Unfortunately, most babies with Edwards or Patau syndrome will die during the pregnancy or shortly after birth.
Screening Tests versus Diagnostic Tests
Both NIPT and nuchal translucency are screening tests. Screening tests tell you how likely it is that your baby has a chromosomal problem. The test result will be either low risk or high risk.
Screening tests cannot 100% exclude or confirm the problem. If your test result is high risk, I’ll discuss diagnostic testing (amniocentesis or CVS) with you.
Non-Invasive Prenatal Testing (NIPT)
During pregnancy, some of the baby’s DNA crosses the placenta into the mother’s blood. The lab takes a sample of your blood, finds the baby’s DNA and works out the number of chromosomes.
NIPT screens for Down syndrome, Edwards and Patau syndromes. It also checks for abnormalities of the sex chromosomes (X and Y). This means NIPT will test the baby’s gender. You can choose whether or not to have the gender revealed!
NIPT will detect 99.5% of Down syndrome cases (it will only miss 1-in-200 cases). It is performed any time after 10 weeks of pregnancy. The test carries no risk to the baby.
One small disadvantage is that NIPT doesn’t tell us anything about the baby’s anatomy (because it’s just a blood test). I recommend you have an ultrasound scan between 12 and 15 weeks to make up for this.
Nuchal Translucency (NT ) Screening
With the introduction of the NIPT, fewer women are choosing nuchal translucency screening. However, it’s still a very good test. We also use NT in pregnancies where NIPT isn’t suitable. There are 2 parts to the NT screening test:
- A blood test
- A special ultrasound scan, which must be performed between 11+0 and 14+0 weeks by a certified practitioner (I am certified by the Fetal Medicine Foundation and have performed thousands of NT scans over the years).
Nuchal translucency detects approximately 95% of cases of Down syndrome. It will miss 1-in-20 cases (a higher number than are missed on NIPT). Because it involves a detailed ultrasound scan, it can also detect early problems with the baby’s anatomy. It is non-invasive, so it carries no risk to the baby.
One small disadvantage is that NT does not tell you the baby’s gender (but this can be checked at the 18-20 week fetal anatomy scan).
Firstly, you don’t have to have these tests. Some women choose not to have any screening, because the result wouldn’t make any difference to how they approached the pregnancy.
On the other hand, some women feel they need even more certainty than a screening test can offer. After careful counselling about the risks, these women can choose to go straight to diagnostic testing.