Amniocentesis is a procedure in which amniotic fluid is aspirated through a percutaneously inserted needle. This procedure is used for a number of diagnostic and therapeutic purposes. One diagnostic application, in the early to mid second trimester, is to determine the fetal karyotype. The karyotype is assessed by culturing fetal cells that have been shed into the amniotic fluid. Another diagnostic application of amniocentesis is to test for fetal lung maturity in the third trimester. On the therapeutic side, amniocentesis is performed to reduce the amniotic fluid volume in a number of situations, including relieving maternal symptoms in a pregnancy complicated by polyhydramnios and treating twin–twin transfusion syndrome by taking fluid from the recipient twin’s sac.

Risks of amniocentesis include amniotic fluid leak, chorioamnionitis, and intra-amniotic hemorrhage. There also appears to be a small added risk of fetal demise, with the pregnancy loss rate after second-trimester amniocentesis reported to be approximately 0.4% above the background loss rate.

Ultrasound plays a number of important roles when performing amniocentesis. Before the needle is inserted, ultrasound is used to select a site that permits safe access to the amniotic fluid, avoiding the fetus, umbilical cord, large uterine blood vessels, and, if possible, the placenta. Real-time ultrasound guidance is used to direct the needle into the uterus (Figure 25.1.1). Continuous real-time monitoring is used throughout the procedure in case movement of the fetus toward the needle (Figure 25.1.2) or uterine contraction (Figure 25.1.3) requires the needle position to be changed.

If the needle traverses the placenta, blood is often seen streaming from the placenta into the amniotic fluid as soon as the needle is removed (Figure 25.1.4). This placental bleeding usually stops within a short time and carries no sequelae, especially with amniocenteses performed before the third trimester.

When a patient develops a fluid leak per vagina after amniocentesis, the sonogram will often demonstrate chorioamniotic separation (Figure 25.1.5). If the hole in the amnion closes, the leakage will resolve and the fluid between the amnion and chorion will disappear on follow-up scans.