Good prenatal care often involves screening and diagnostics tests, both for you and your baby. Here, we explain the 5 most common such tests for your baby.
Nuchal Translucency (NT) Screening Test
The NT test is an ultrasound method performed between the 11th and 14th weeks of pregnancy. In this test, the doctor measures the amount of translucent fluid at the back of your baby’s neck and, if it is higher than normal, it may indicate either a chromosomal problem, such as Down syndrome or Trisomy 18, or certain congenital heart defects. The higher the measurement, the more likely it is that there is a problem. However, this test is not definitive as it only reveals a probability that your baby may have a problem. Also, many babies who have a thicker-than-normal NT turn out to be perfectly healthy! The only way to know for sure if your baby has a chromosomal issue is to take a diagnostic test such as chorionic villus sampling (CVS) test, amniocentesis, or cell-free DNA testing.
Chorionic Villus Sampling (CVS) Test
The CVS test is a diagnostic procedure that involves removing some chorionic villi cells from the placenta at the point where it attaches to the uterine wall. CVS is usually performed between the 10th and 13th weeks of pregnancy. The CVS test can detect chromosomal abnormalities, genetic disorders (such as cystic fibrosis) but, unlike amniocentesis, it does not detect neural tube defects. The level of accuracy of a CVS test is high – around 98–99% – but it does not assess the severity of these health problems.
Although CVS testing is generally considered to be safe, it is an invasive test and, therefore, it carries risks. The most important one is miscarriage, which is thought to occur in 1 out of every 100 procedures. The advantage of CVS testing over amniocentesis is that it can be performed earlier. This allows you to prepare yourself better if your baby has a problem and you decide to continue with the pregnancy. Conversely, if you choose to interrupt the pregnancy, earlier will be less traumatic than later (the amniocentesis is usually performed at around the 20th week of gestation).*
Non-Invasive Prenatal Testing (NIPT)
NIPT, also known as cell-free DNA screening, is a diagnostic test for specific chromosomal abnormalities in a developing fetus. NIPT differs from CVS testing and amniocentesis in that it doesn’t require the extraction of cells (whether from the placenta or the amniotic fluid) and only requires the pregnant woman to give a blood sample, which will naturally contain fetal DNA. Consequently there is no risk of miscarriage. However, this test has limits and may result in a higher rate of false-positive results, compared with more traditional diagnostic tests. Therefore NIPT is not recommended for the general population but is recommended for:
- Women with Rh negative blood type
- Women who are carriers of an X-linked recessive disorder
- Women with risk factors for having a baby with chromosomal disorders.
Nuchal Fold Test
This test is similar to the nuchal transparency test in that it is also conducted with ultrasound and measures an area at the back of the neck. However, this test is performed between the 16th and 24th weeks of pregnancy and, by this time, the area behind the neck is no longer translucent. This measurement screens for the same abnormalities but has a higher threshold of what is considered normal, compared with the nuchal translucency test.
The fetal non-stress test is performed from the 28th week of pregnancy onwards. It involves attaching a belt to the mother’s abdomen to measure the heart rate and another belt to measure contractions. Movement, heart rate and activity are then assessed over a 20–30 minute period. This test is used to make sure the fetus is getting enough oxygen – low oxygen means the placenta or umbilical cord may not be functioning properly. Babies getting enough oxygen will respond with an increased heart rate during times of movement and the heart rate will decrease at rest. This is called a reactive test and shows the baby is getting enough oxygen. A non-reactive text, where the heart rate doesn’t change during movement or the baby isn’t moving suggests inadequate levels of oxygen in the baby. However, additional testing is required to confirm this. For all we know, the baby may simply be having a sleep!
*Amniocentesis, whereby cells are taken from the amniotic fluid, would usually be mentioned as a common diagnostic test. However, CVS testing and NIPTis are becoming increasingly more common and, consequently, the proportion of women undergoing amniocentesis is steadily declining.