Prenatal Genetic Screening
by Tracy B. Wright, M. D
As part of your routine prenatal care, we counsel patients daily on their options for prenatal genetic screening for certain chromosomal abnormalities. We are required to inform every pregnant patient of their options. I always counsel patient to consider how they will use the results of these optional tests before making their decision. There are 3 prenatal genetic screening tests, first trimester screening, second trimester screening, and noninvasive prenatal testing. These tests screen for 2 or 3 of the most common trisomies (3 copies of one chromosome): Trisomy 21 (Down Syndrome and the most common), trisomy 18, and trisomy 13. There are also 2 traditional diagnostic tests, chorionic villus sampling and amniocentesis.
First trimester screening can be performed at 11 – 14 weeks’ gestation, and screens for trisomies 18 and 21. This test involves obtaining a maternal blood sample and measuring 4 substances released by the pregnancy cells. This may be combined with measurement of the skin fold behind the baby’s neck (nuchal thickness). The level of these substances and the nuchal thickness are put into a formula in combination with the mother’s age to determine a risk assessment. There is a 90% detection rate. Second trimester screening, also called multiple marker screening or quadruple screening, is performed at 15-21 weeks’ gestation, and screens for trisomies 18 and 21, and spinal cord abnormalities. This test also involves obtaining a maternal blood sample and measuring a different set of 4 substances released by the pregnancy cells. In a similar fashion, the level of these substances are put into a formula in combination with the mother’s age to calculate a risk assessment. The detection rate for the second trimester screening is 80%. For both the first and second trimester screenings, a negative screen = low risk, and a positive screen = higher risk. I always advise my patients that with both the first and second trimester screenings, a negative screen means the risk of having a child with a chromosomal abnormality is very low, but not zero. We recommend additional genetic counseling and for patients to consider diagnostic testing for a positive screen.
Noninvasive prenatal screening (NIPT) is the newer option for mothers that can be performed as early as 10 weeks’ gestation. This again involves obtaining a maternal blood sample from which fetal DNA fragments can be extracted to screen for trisomies 13, 18, and 21, as well as sex chromosomal abnormalities. The detection rate for trisomies 18 and 21 is > 95%, and for trisomy 13, 87%. This test is rapidly replacing the more invasive diagnostic tests, chorionic villus sampling and amniocentesis. However, the NIPT is also a screening test, and these diagnostic tests are recommended if a chromosomal abnormality is detected. Chorionic villus sampling (CVS) involves obtaining a sample of the placenta through the cervix or abdomen with ultrasound guidance, and amniocentesis involves obtaining a sample of amniotic fluid through the mother’s abdomen, also with ultrasound guidance. The baby’s chromosomes can be directly examined from both of these samples. Although both procedures are very safe, with 99.9% detection rate, they are also invasive, with 0.5 – 1% complication rate that could include bleeding, infection, and loss of the pregnancy.
Insurance coverage for all of these tests, except the second trimester screening, varies depending on the insurance carrier and specific insurance plan. The second trimester screening is almost always covered. The other tests are more likely to be covered for patients with increased risks to include mothers over age 35, abnormal findings on the baby’s anatomy scan, and patients with a prior history of a child with chromosomal abnormalities.