Amniocentesis is a procedure in which a small amount of amniotic fluid is obtained and then analyzed. The best time to do an amniocentesis is between 16 and 17 weeks. Chromosome studies are done on cells contained in the fluid for conditions such as Down syndrome. Amniotic fluid also assists in screening for neural tube defects, such as spina bifida and genetic disorders such as sickle cell anemia.
An ultrasound examination is done first to date the pregnancy and locate the placenta and the amniotic fluid surrounding the baby. With ultrasound guidance, a needle is inserted through the mother’s abdominal wall into the amniotic fluid. A small amount of fluid is removed and sent for analysis. The entire procedure usually takes just a few minutes.
Some women say the procedure is uncomfortable, although many women say the sensation is like a routine blood draw. If desired, a local anesthetic can be used to numb the skin before the needle is inserted. Some women do experience cramping after the test. Most women are able to return to their regular activities the day after amniocentesis.
Some women experience vaginal fluid loss, cramping, and a small amount of vaginal bleeding. This should resolve quickly and go away without further complications. The following is recommended after undergoing amniocentesis:
There is a natural miscarriage rate throughout any pregnancy. Amniocentesis increases this natural complication rate for this period of pregnancy by less than .02 to .03 percent. That is, less than 1 in 300 to 500 women would have a miscarriage as a result of amniocentesis.
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Careful study of the cells contained in the amniotic fluid will identify chromosome problems, such as Down syndrome. Measurement of the protein in the amniotic fluid, called Alpha Fetoprotein (AFP), will detect the majority of neural tube defects, such as spina bifida. When a history of genetic disease is known in a family, specific testing may be available through DNA or enzyme testing.
No method of prenatal testing can guarantee that a baby will be born without birth defects, genetic disease or mental retardation.
Results of the chromosome study and AFP measurement are usually available in less than two weeks. Special studies may take longer.
Chromosome results are greater than 99 percent accurate and measurement of the AFP detects approximately 95 percent of neural tube defects. DNA and enzyme studies also have high accuracy.
Occasionally, test results need to be clarified through further ultrasound evaluations or blood tests on both parents. A repeat amniocentesis or fetal blood sampling is rarely needed.
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