Prenatal Genetic Testing

Chromosomal abnormalities occur in about 1 in 200 live births, although most fetuses with such abnormalities die before birth. Factors increasing this risk include pregnancy after the age of thirty-five, previously having a child or fetus with a birth defect, and a family history of chromosomal abnormalities. A number of common tests are available to screen for or diagnose genetic abnormalities in the fetus. After the third month of pregnancy, ultrasound can detect whether the fetus has any obvious structural defects.

Removal of amniotic fluid for medical examination goes back to the late 1870s. In 1956, John H. Edwards discussed the use of amniotic fluid obtained from amniocentesis for the “antenatal detection of hereditary disorders.” This fluid surrounds the fetus and contains cells used to prepare a karyotype, which displays chromosomes arranged in pairs. This procedure is commonly performed between the fifteenth and twentieth weeks of pregnancy to detect such conditions as Down syndrome (trisomy 21) —in which there is an extra chromosome 21--spina bifida, cystic fibrosis, and Tay-Sachs disease.

An alternative diagnostic procedure to amniocentesis is chorionic villus sampling (CVS), which is usually performed during the tenth and twelfth weeks of pregnancy, thus the results are determined much earlier. CVS, first performed in 1983 by the Italian biologist Giuseppe Simoni at the Biocell Center, can detect more than 200 genetic abnormalities. The chorion is a portion of the fetal membrane that forms on the fetal side of the placenta; the chorionic villi are small, finger-like projections in the chorion that are removed for study during the procedure. Since the villi are of fetal origin, they can provide a sample of the genetic makeup of the fetus.

There are two types of tests that can be conducted during pregnancy for Down syndrome: a screening test using ultrasound (sonogram), which gives an indication that there is a higher risk of a disorder; and a diagnostic test, such as amniocentesis or chorionic villus sampling, which provides a firm diagnosis.

In 2011, cell-free fetal DNA tests became available. Unlike amniocentesis and CVS, the cell-free tests are noninvasive and only involve a blood test about the tenth week of pregnancy. Since they only evaluate DNA fragments in the bloodstream, unlike the older tests, cell-free is only a screening test (as for Down syndrome) and not a diagnostic test of a genetic defect.