amniocentesis

Amniocentesis is a prenatal diagnostic procedure in which a small amount of amniotic fluid is drawn from the amniotic sac for laboratory testing. The fluid contains fetal cells and biochemical substances that can reveal genetic and developmental conditions. The procedure is guided by ultrasound and most often performed after 15 weeks of gestation.

Indications include advanced maternal age, abnormal prenatal screening results, a known fetal genetic disorder, or a history of affected pregnancies. It may also be used to determine fetal sex when required to assess risk for X-linked conditions.

The procedure involves cleaning the abdomen, using local anesthesia if needed, and inserting a thin needle through the abdominal and uterine walls into the amniotic cavity under ultrasound guidance. A small volume, typically 20 to 30 milliliters, is withdrawn and sent to the laboratory. The procedure is usually brief and followed by a short observation period.

Laboratory testing can include fetal karyotyping, targeted genetic tests, and biochemical assays such as alpha-fetoprotein. Results for karyotype and most genetic tests typically take one to two weeks; rapid testing is available for some conditions. Risks include miscarriage (about 0.1% to 0.3%), fetal injury, infection, and fluid leakage. Some pregnancies experience cramping or light vaginal bleeding. Limitations include the possibility of nondetection of certain conditions and mosaicism. Alternatives include chorionic villus sampling in the first trimester and noninvasive prenatal testing from maternal blood.