Genetic Screening Tests in Pregnancy

Human cells contain 46 chromosomes (23 pairs), the building blocks of our genetics. A chromosome count in cells other than 46 is called “aneuploidy.” An aneuploid pregnancy can occur in any family. Embryos with aneuploidy resulting from errors during fertilization are usually lost through miscarriage in the very early weeks of pregnancy. Some types of aneuploidy are compatible with life, but in these cases the baby has serious problems in many organ systems, most notably intellectual disability. Certain tests performed during pregnancy can screen for these conditions. The primary purpose of these tests is to screen for Down syndrome, the most common of these conditions among live births, which is caused by three copies of chromosome 21. Down syndrome is the most common condition that is compatible with life and causes varying degrees of intellectual disability. Individuals with Down syndrome may also have abnormalities of the heart and circulatory system, digestive system and central nervous system. It occurs in about 1 in 700 live births, and the risk of having a baby with Down syndrome increases with advanced maternal age.

These screening tests, described in detail below, provide a risk ratio for certain aneuploidies and give the family the opportunity to decide early on whether to undergo procedures for a definitive diagnosis. The screening tests that may be performed are, in order:

– Combined (Double) Test: Performed between weeks 11 and 14 of pregnancy. First, the thickness of the translucent layer at the back of the baby's neck (nuchal translucency) is measured by ultrasound, and on the same day a blood sample is taken from the mother to check the levels of certain hormones. All results are evaluated together to determine the likelihood of Down syndrome in the baby. Increased nuchal translucency and a high-risk result do not mean that the baby definitely has a disorder. Similarly, normal nuchal translucency and a low-risk result do not mean that the baby is definitely healthy. This test detects about 85% of Down syndrome cases.

– Triple and Quadruple Tests: These tests are performed between weeks 15 and 20 of pregnancy. Because they do not use an ultrasound parameter of the baby in the risk calculation, they should not be the first choice. The levels of certain hormones are checked in a blood sample taken from the mother. The triple test detects about 65% of Down syndrome cases and the quadruple test about 85%.

– Cell-free fetal DNA testing in maternal blood (NIPT): These tests can be performed after week 10 of pregnancy, and there are several types using different methods. By detecting the baby's genetic material in a blood sample taken from the mother, many genetic disorders, primarily aneuploidies, can be screened. The Down syndrome detection rate is about 99%.

Once the screening test to be performed has been decided together with the family, the necessary examination and counseling will be provided by your doctor.