Prenatal screening: the facts

So you’re thinking about having prenatal screening. A few things you might consider…. Why do you want to have screening done and what will you do if you get an abnormal result?

There are a number of options for prenatal screening. The first thing to understand is the difference between a screening and a diagnostic test. A screening test gives you a result that will show your risk or a percentage likelihood of your baby being affected with the condition. Screening tests include nuchal translucency scan and bloods (PAPP-A and bHCG) or non-invasive prenatal testing (NIPT) which includes panorama and harmony test. A diagnostic test gives you a yes or no answer. These test are an amniocentesis and chorionic villus sampling tests.

First trimester screening involves a risk assessment based on your age and medical history. This is imputed into a computer package along with blood test results (PPA-A and bHCG) and the nuchal translucency. Nuchal transluceny is the measurement of fluid at the back of your baby’s neck that is performed by ultrasound scan between 11 and 14 weeks of pregnancy. A low risk result is >1 I 250. For example, a 30 year old mother has an age related risk of 1 in 1,000 (low risk). After the test the result may show a high-risk result (1 in 100). If this happens than a diagnostic test is recommended (see below). However is the result after the test is 1 in 2000 she is still low risk. But this still means that her baby could be the one affected.

NIPT: A blood test is taken from the mother to check the baby’s chromosomes (baby’s DNA circulating in the mother’s blood). There are two versions of NIPS, panorama and harmony. Panorama can be performed from 9 weeks in a singleton pregnancy and harmony from 10 weeks of pregnancy. The results check for abnormal chromosome conditions including Down syndrome, Edward syndrome, Patau syndrome and Turner syndrome. The results are usually available after 10 working days. The tests usually cost between 450 and 500 euro.

Chorionic villus sampling is when a sample of the placenta is taken by passing a needle through the mother’s abdomen under ultrasound guidance. The sample is sent to the lab and an initial result is available after 48 hours with the full result taking about two weeks. This test has a 1% risk of miscarriage associated with it. An amniocentesis is another needle test, which samples the amniotic fluid after 15 weeks gestation. It has a 0.5% risk of miscarriage associated with it. Both of these are diagnostic tests and are recommended when a screening test gives a high-risk result or when an abnormality is detected on the 20 week anatomy scan.

So I would recommend NIPT (panorama or harmony), which are both available in Ireland. They are superior to first trimester screening for women in their 30s and 40s. Discuss them with your obstetrician. Make an informed choice.