Parents often want reassurance that their child is normal before birth. Sometimes, they have specific concerns about genetic diseases that run in their families or that are related to advanced maternal age, such as Down syndrome. Fetal genetic testing can be done in several ways, beginning before embryos are even implanted in the womb.
Pre-implantation Genetic Diagnosis (PGD)
PGD is technically performed on embryos, not fetuses. It's the earliest diagnostic test for potential fetal abnormalities and can be done only in cases of in vitro fertilization, because embryos are tested before they're put into the womb. PGD is expensive and can be done only in select laboratories. A single cell is removed from a developing embryo; loss of one cell does not harm the embryo in any way. A variety of known genetic defects can be tested for, but PGD does not analyze for all possible genetic anomalies. Results are obtained within a few days. PGD has been criticized for its potential; parents could select certain characteristics to create "designer babies," although the technology is not yet that advanced. Sex selection, however, is possible.
Chorionic Villus Sampling (CVS)
In chorionic villus sampling (CVS), cells are removed from the chorionic villi, tissue from part of the placenta. Cells removed are fetal cells that can be tested for a number of genetic anomalies. CVS is done between weeks 10 and 14; earlier testing has been associated with limb malformations, according to the Merck Manual. A needle is passed through the cervix or through the lower abdominal wall under ultrasound guidance to obtain the sample. A small amount of cramping and spotting is normal after the procedure, the Mayo Clinic states. Test results take 10 to 14 days to obtain.
Amniocentesis
Amniocentesis is one of the earliest methods of fetal genetic testing. A needle is inserted through the mother's abdomen into the amniotic fluid, and a sample is withdrawn and analyzed. Amniocentesis is done after 14 weeks; earlier testing is associated with club foot malformations and fetal loss, the Merck Manual states. If the procedure is done after 14 weeks, fetal loss is rare, less than 1 percent. Test results take 10 to 14 days to obtain.
Maternal Serum Screening Tests
Tests can be done on a mother's blood to screen for genetic problems, such as Down syndrome and other trisomies, or neural tube defects, in which the spinal cord isn't formed correctly. Screening is done between 15 and 20 weeks. These screens are not 100 percent accurate and cannot test for all known abnormalities. A quadruple screen test, which tests alpha-fetoprotein levels, estradiol, human chorionic gonadotropin and inhibin-A, can be accurate in detecting Down syndrome (80 percent reliability), neural tube defects (75 to 80 percent reliability) and anencephaly, or lack of the fetal brain (95 percent reliability), according to the March of Dimes. Because these tests are non-invasive and pose no risk to the fetus, the American Congress of Obstetricians and Gynecologists recommend all pregnant women be screened.
