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What does amniocentesis show?
Amniocentesis is very accurate in detecting chromosomal abnormalities and the gender of the fetus. Occasionally, the fetal cells harvested by amniocentesis do not grow in the culture, and no chromosomal data can be obtained. Additionally, physician may be unable to extract fluid from the uterus during amniocentesis due to a variety of technical reasons. In both instances, a subsequent extraction of fluid may be attempted.
Amniocentesis testing for lung maturity can help the doctor make decisions regarding timing of delivery for women in premature labor, those with diabetes, or other medical conditions requiring possible early delivery. Lung maturity testing is not perfect. In rare instances, a baby with lungs that are predicted to perform normally can still experience difficulty breathing.
The AFP (alpha fetoprotein) blood test is currently available and can be used to screen for neural tube defects such as spina bifida, as well as Down's syndrome. An elevated AFP blood level may indicate the possibility that the fetus has a defect in the brain and/or spinal cord development. A low AFP level may indicate the potential for Down's syndrome. When abnormal blood levels of AFP are found, further evaluation with ultrasound and amniocentesis can be done to look for birth defects and chromosomal abnormalities.
Amniocentesis can also help determine if there are specific genetic problems present in the fetus. Testing for Tay-Sachs disease, cystic fibrosis, sickle cell anemia, and other inherited disorders can be performed on samples of amniotic fluid. Checking the cells for specific genes can now be done on the fetal cell cultures. It is also possible to look for a specific protein in the amniotic fluid that can signal a genetic abnormality. Testing for genetic (inherited) diseases is not routinely performed, but is available when necessary.
Another common reason for performing amniocentesis is to determine if the fetal lungs are mature enough so that the baby can be safely delivered before the due date. If the fetal lungs are not mature, an attempt can be made to stop premature labor with medications. If the lung tissues are mature, it may be safe for the premature baby to be born. This test can also help in the timing of delivery when the due date is uncertain.
Occasionally, a pregnant woman may have a fever without an obvious site of infection. In these instances, an amniocentesis may be performed to determine if the cause of the mother's fever is from an intrauterine infection.
The overall possibility of birth defects in any pregnancy is approximately 3%. For defects such as cleft lip, hernia, and extra fingers or toes, amniocentesis will not be diagnostic. Ultrasound may be helpful in detecting these defects in the fetus.