All women who become pregnant are at risk for having an infant affected by certain genetic disorders caused by abnormal chromosomes. The most common of these disorders are Trisomy 21, also known as Downs Syndrome, Trisomy 18, and Trisomy 13. The risk of having an infant affected by one of these disorders is related to the maternal age. The older a woman is, the more likely she is to have an infant with one of these types of disorders. Nevertheless, all women regardless of age are offered screening or diagnostic testing to help determine if the pregnancy is normal. Genetics testing can be offered in several ways.
First Trimester Nuchal Translucency Testing – This test, offered between 11 and 14 weeks gestation, uses an ultrasound to measure the fluid at the back of a baby’s neck, and blood work form the mother to measure levels of certain blood proteins. The combination of these is used to predict the likelihood that the pregnancy is affected by chromosome abnormalities such as Down syndrome, Trisomy 18, and Trisomy 13, and will detect up to 85% of affected pregnancies. Remember that this blood test only tells you if you are at risk for having a baby with any of these disorders. Follow-up testing is needed to confirm the presence of these disorders.
Second Trimester Penta Screen - This series of blood tests is usually offered between the 15th and 20th week of pregnancy screens for genetic disorders as well as Currently, AFP is combined with measurements of estriol , inhibin invasive trophoblast antigen (ITA), and HCG levels to screen for Down’s syndrome and other chromosome abnormalities like trisomy 13 and trisomy 18. AFP is often elevated in the blood of mother’s carrying a baby with a neural tube defect in which the spinal cord does not close normally. If the AFP is low, and if the other tests are low and high respectively, an increased risk of Down syndrome is present.
Much controversy surrounds genetic screening since the tests can produce false-positive results. At times, women who are carrying normal fetuses will be unnecessarily alarmed and advised to undergo further testing based on an initial screening.
While the above testing helps to screen and identify those individuals who are at higher risk for carrying an infant with a genetic syndrome, confirmatory testing must be performed to identify whether a genetic problem actually exists and what specific genetic rearrangements are present. Your physician may offer you the following tests to identify these problems.
Chorionic Villus Sampling (CVS) - CVS was developed in the 1970’s as an alternative to amniocentesis to detect genetic or chromosomal abnormalities. It can be performed earlier in the pregnancy (usually between the 9th-13th weeks) than amniocentesis, thus providing earlier results. CVS involves the sampling of chorionic cells obtained either by placing a catheter through the cervix or through the abdominal wall. The results can provide a complete picture of the genetic make-up of the developing fetus. CVS does slightly increase the risk of both miscarriage and infection, so it is important to weigh the risk/benefit ratio before consenting to this procedure. Your doctor can help you in evaluating your individual risk for genetic or chromosomal abnormalities.
Amniocentesis - Amniocentesis is a procedure in which amniotic fluid is taken from the uterus using a long needle that is inserted through the abdominal wall. It is usually performed between the 16th –18th week of pregnancy and it is currently the most popular technique used to screen for genetic and chromosomal disorders. Amniocentesis is preceded by an ultrasound exam to determine the baby’s position and the location of pockets of amniotic fluid. Once the amniotic fluid is obtained, the cells are cultured and stimulated to grow for one to two weeks. After that, the cell chromosomes can be examined and test results determined. Amniocentesis does carry a slight risk of miscarriage (about 1 in 200-400) so the risk/benefit ratio of the test must be considered before consenting to this procedure