Amniocentesis

Genetic amniocentesis is typically performed at 14 to 20 weeks of gestation but can also be done any time after 20 menstrual weeks. After ultrasound examination of the fetus and placenta, an area of skin overlying a pocket of amniotic fluid is cleaned with iodine solution. With ultrasound guidance throughout the procedure, a 22-gauge spinal needle is used to remove 20 mL of amniotic fluid. Special care is taken to avoid the fetus, umbilical cord, and the large placental vessels. In experienced hands, the pregnancy loss rate attributed to the procedure is about 1 in 300. The entire testing time for a chromosomal analysis is about 10 to 12 days. Alternately, the supernatant may be assayed for metabolites to diagnose other disorders that run in the family if identified on counseling. Earlier amniocentesis (at 11 to 13 weeks) has been successfully performed but is not recommended any longer because of the recent initial reports regarding a slightly higher rate of clubfoot in these newborns.

EVIDENCE-BASED SUMMARY

• Early amniocentesis performed before 14 completed weeks of gestation is not considered a safe alternative to second-trimester amniocentesis or chorionic villus sampling (CVS) (SOR: A).

• CVS should not be performed before 10 completed weeks of gestation (SOR: B).

• CVS should always be performed under direct ultrasound control (SOR: B).

• Third-trimester amniocentesis does not appear to be associated with significant risk of complications leading to emergency delivery (SOR: B).

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