This is the most common prenatal diagnostic procedure. T ransabdominal withdrawal of amnionic luid is generally done between 15 and 20 weeks but may be performed at any point later in gestation. Indications include diagnosis of fetal genetic disorders, congenital infections, and alloimmunization, as well as assessment of fetal lung maturity. The most common types of prenatal diagnostic tests are CMA to assess copy-number gains or losses, karyotype analysis to test for aneuploidy, and FISH to identiY gain or loss of specific chromosomes or chromosome regions (Chap. 13, p. 270). Because amniocytes must be cultured before fetal karyotype can be assessed, the time needed for karyotyping is 7 to 10 days. In contrast, FISH studies are usually completed within 24 to 48 hours. CMA can oten be performed directly on uncultured amniocytes with a turnaround time of only 3 to 5 days, and if amniocyte culture is required, turnaround time is 10 to 14 days (merican College of Obstetricians and Gynecologists, 20 16b).
Amniocentesis is performed using aseptic technique, with a 20or 22-gauge spinal needle and ultrasound guidance (Fig. 14-4). A standard spinal needle is 9 em long, and depending on patient habitus, a longer needle may be required. Measurement of the
TABLE 14-12. Selected Tests Performed on Amnionic Fluid and Typical Volume of Fluid Required
FIGURE 14-4 A. Amniocentesis. B. The amniocentesis needle is seen in the upper right portion of this sonogram. (Reproduced with permission from Mastrobattista JM, Espinoza J: Invasive prenatal diagnostic procedures. In Yeomans ER, Hoffman BL, Gilstrap LC III, et al (eds): Cunningham and Gilstrap's Operative Obstetrics, 3rd ed. Ne
York, McGraw-Hili Education, 201o7.) sonographic distance from skin to amnionic luid pocket may aid needle selection. Sonography is used to identiY a pocket of amnionic luid that is close to the midline, being cognizant of uterine size and shape. he needle is inserted perpendicular to the skin and guided into the deepest portion of the luid pocket, avoiding fetal parts and umbilical cord. Eforts are made to puncture the chorioamnion rather than to push or "tent" it away from the uterine wall. he amnion usually uses with the adjacent chorion by 16 weeks' gestation, and the procedure is generally deferred until ater chorioamnion usion has occurred. Discomfort from the procedure is considered minor, and local anesthetic has not been found to be beneicial (Mujezinovic, 2011).
Following the procedure, the color and clarity of the luid are documented. Amnionic fluid should be clear and colorless or pale yellow. Blood-tinged luid is more frequent if there is transplacental passage of the needle. However, it generally clears with continued aspiration. The placenta implants along the anterior uterine wall in approximately half of pregnancies. In these cases, the placenta will be traversed by the needle approximately 60 percent of the time (Bombard, 1995). Needle passage through the placenta is avoided when possible, although fortunately this has not been associated with greater pregnancy loss rates (Marthin, 1997). Dark brown or greenish luid may represent a past episode of intraamnionic bleeding.