There are several different options available to pregnant women to test for certain genetic conditions and birth defects, including Down syndrome, Trisomy 18 and open neural tube defects. Down syndrome is caused by an extra chromosome #21 and results in both mental disabilities and physical abnormalities. Trisomy 18 is caused by the presence of an extra chromosome #18 and causes severe mental retardation and birth defects. Most children with Trisomy 18 do not survive the first year of life. The risk of chromosomal abnormalities like Down syndrome and Trisomy 18 increases with maternal age. Neural tube defects occur when the neural tube, which develops into the baby’s spinal cord and brain, does not close completely. This is also known as spina bifida. This leaves an opening along part of the baby’s spine or head and can cause paralysis and other physical or mental problems. Recent studies have shown that prenatal surgery can possibly correct these defects or reduce the rate of paralysis with these defects in certain cases. The risk of open neural tube defects is not affected by maternal age.
The tests available to pregnant women include screening tests and diagnostic tests. Screening tests are non-invasive tests that better identify a woman’s personal risk of carrying a child with one of these conditions. Screening tests most often provide reassuring results and many women choose not to have any further testing if the screening comes back negative or at low risk. Diagnostic tests are invasive procedures that will tell her with certainty whether her child has these conditions. A woman can also choose to not have any testing performed.
A second trimester only test is also available, called the AFP4. This non-invasive test measures 4 hormones in the maternal blood and detects 81% of fetuses with Down syndrome.
A targeted high-resolution ultrasound performed by the maternal-fetal medicine specialists can screen for birth defects or fetal anomalies, as well as identify placental, uterine or amniotic fluid abnormalities. Fetal ultrasound may not detect all fetal anomalies. Between 50% and 75% of fetuses with Down syndrome will have a visible ultrasound abnormality, so a normal ultrasound does not guarantee that Down syndrome is not present. More than 95% of fetuses with Trisomy 18 and spina bifida will have an ultrasound abnormality.
There is a screening option available that screens for open neural tube defects only. This is a blood test that measures the maternal serum alphafetoprotein level. It is measured during the second trimester, and if it is elevated there is an increased risk of open neural tube defects or other pregnancy complications.
If any screening test comes back abnormal, the results and options are discussed with the pregnant woman. She is referred to the maternal-fetal medicine specialists for further counseling and for diagnostic testing options. Diagnostic testing options depend on the stage of the pregnancy. Amniocentesis is generally performed in the second trimester between 15 and 20 weeks. A needle is passed through the abdomen and uterus into the amniotic sac around the fetus and a small amount of amniotic fluid is withdrawn. The fetal cells in the fluid are analyzed for chromosome abnormalities. Other inherited diseases can be tested for as well, such as cystic fibrosis and sickle cell anemia, if the parents are known to be carriers for the disease. The risk of a major complication from the procedure that could lead to miscarriage is 1 in 200, and is often even lower when done by experienced individuals.
Diagnostic testing can also be performed in the first trimester. Chorionic villus sampling (CVS) is performed between 10 and 12 weeks. This procedure obtains a sample of placental tissue to analyze for chromosome abnormalities. A catheter is passed through the cervix or a needle is used through the abdomen to obtain the tissue. The risk of complication such as miscarriage with this procedure is 1 in 100.
Any woman who has had abnormal screening tests will be given the option to proceed with diagnostic testing to determine if the fetus truly has an abnormality. Because screening tests are not 100% accurate in detecting Down syndrome, Trisomy 18 or open neural tube defects, all women may undergo a diagnostic procedure such as amniocentesis or CVS if they so choose.