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When should amniocentesis be performed?
For genetic testing, chromosome analysis, and evaluation of an abnormal AFP test, amniocentesis is usually performed between the 15th and the 21st weeks of pregnancy. There is a higher rate of miscarriage associated with amniocentesis that is performed earlier than the 15th week. There is no time limit as to how late in the pregnancy amniocentesis can be performed.
If amniocentesis is performed to determine fetal lung maturity, it is often done anywhere between the 32nd and 36th week of pregnancy. In women with diabetes, lung maturity testing may be done as late as the 39th week, because poorly controlled diabetes may delay fetal lung maturity.
Is amniocentesis safe?
Overall, amniocentesis is a safe procedure with the risk of fetal loss generally less than 1%. According to the American College of Obstetricians and Gynecologists Practice Bulletin, there is a procedure-related rate of fetal loss ranging from 1/300 to 1/500 procedures.
Because there is still a slight possibility of pregnancy loss from the procedure, amniocentesis should be performed only when clearly indicated and its benefits have been determined to outweigh any associated risks. Many parents ask for the procedure to "reassure" themselves that everything is perfect. It should be emphasized that an amniocentesis with normal results does not guarantee that the baby will not have a birth or genetic defect. Many birth defects cannot be detected by amniocentesis. Besides fetal loss, other risks associated with amniocentesis include bleeding, cramping and leakage of fluid from the amniotic sac. These problems occur in about 1% of women undergoing the procedure, and they generally resolve on their own. If these symptoms arise following the procedure, the woman is often advised to stop work and remain at bed rest until the symptoms cease. Subsequently activity may be restricted by the doctor. Occasionally, the needle enters the fetus during an amniocentesis. This is generally harmless, but the infant should be monitored for a short time following the testing.
Amniocentesis is a relatively safe and painless procedure that can provide helpful information. The procedure can be offered to selected women after reviewing the anticipated risks and benefits involved.
Medically reviewed by Wayne Blocker, MD; Board Certified Obstetrics and Gynecology
ACOG Practice Bulletin No. 88, Dec. 2007. Invasive Prenatal Testing for Aneuploidy. Obstet Gynecol. 2007 Dec;110(6):1459-67.