ANTENATAL TESTS FOR FETAL ABNORMALITIES (NON-INVASIVE AND INVASIVE)
Antenatal tests refer to investigations performed during pregnancy to detect health problems in the growing fetus. These tests are geared to determine if the fetus has any major health conditions that may greatly affect the baby’s quality of life. Prior knowledge of such conditions allows the parents to make decisions about the pregnancy and be better prepared for the delivery of affected babies.
Antenatal tests can broadly be categorised as non-invasive or invasive.
Non-Invasive Prenatal Test (NIPT)
To date, NIPT is the most sensitive screening test for the common chromosomal abnormalities to affect humans like Down syndrome (Trisomy 21), Edward syndrome (Trisomy 18) and Patau Syndrome (Trisomy 13).
DNA from the baby’s placenta called cell-free DNA actually circulates within mother’s blood stream. NIPT involves a simple blood test from the pregnant woman any time after 10 weeks of pregnancy, after which the cell-free DNA is tested to check if the baby is at increased risk of having chromosomal or genetic disorders. NIPT can also reveal the gender of the baby.
Although NIPT is highly accurate (97% to 99% for Down syndrome, Edward syndrome and Patau syndrome), it cannot determine for sure if the baby is affected by the conditions.
If the results of NIPT suggests the fetus has a chromosomal or genetic disorder, a diagnostic test like chorionic villus sampling (CVS) or amniocentesis is still required to confirm the diagnosis.
In our clinic, parents who wish to have NIPT usually have the blood drawn at around 10 weeks of pregnancy. They would then be scheduled to return to the clinic 2 to 3 weeks later, not only to receive the result but to have an assessment of the nuchal translucency, the nasal bone and structural survey of the fetus as well.
Pregnant women considered to have a high risk of carrying a fetus with chromosomal or genetic abnormalities can confirm the diagnosis with diagnostic testing procedures like chorionic villus sampling (CVS) or amniocentesis. These procedures involve removal of placental cells (CVS) or the fluid that surrounds the fetus (amniocentesis). Typically, CVS is done at 10-14 weeks’ gestation and amniocentesis is done at 16-20 weeks’ gestation. The most serious complication of these procedures is miscarriage. For every 100 women who undergo CVS, approximately one woman will have a miscarriage as a result of the procedure. Amniocentesis can also cause miscarriage though the risk is lower than with CVS.