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Over the past 35 years, as patients' expectations concerning pregnancy evaluation have grown, so have physicians' attempts to make fetal assessment as accurate and safe as possible. Advances in prenatal diagnosis have depended on the development and improvement of less invasive diagnostic methods such as ultrasound, serum screening for aneuploidy, and middle cerebral artery Doppler to rule out fetal anemia. But despite these safer techniques, amniocentesis remains an essential tool in prenatal evaluation and diagnosis.
We'll review the indications, techniques, and complications associated with amniocentesis after 15 weeks' gestational age. Because of its unique considerations and complications, early amniocentesis (amniocentesis performed after 10 weeks but before 15 weeks' gestational age) is no longer in vogue and will be covered only briefly.
When is amniocentesis indicated?
Amniocentesis is also done after an abnormal noninvasive screening test-first- or second-trimester serum screening, or U/S imaging-and genetic screening (enzymatic or DNA-based analysis in cases of families with histories of certain inherited disorders). Finally the procedure can help document fetal lung maturity, the rupture of membranes, and the existence of intrauterine infection, and is useful in assessing Rh isoimmunization.
Therapeutic applications of amniocentesis include amnioreduction for polyhydramnios (in cases of twin-twin transfusion syndrome or fetal anomalies). The technique has also been adapted in the development of selective pregnancy reduction and transabdominal chorionic villus sampling.
Developing one's technique
Counsel patients about the benefits and risks of, and alternatives to amniocentesis. They should be aware of the limitations of the procedure, which in the case of amniocentesis for fetal karyotype and other genetic studies, include the inability to screen for all possible genetic disorders, the delay that sometimes occurs in the reporting of results, the possibility that the test will fail to provide results (in the case of a failed procedure or failed cell culture), and the rare possibility that there may be an error in the results.
We occasionally use a longer spinal needle when the patient is obese or for positioning purposes. A 20- or 22-g needle is generally acceptable, but larger-bore needles have been linked to more adverse reactions. Ultrasound probe covers are commercially available, but a less expensive sterile glove works just as well to cover the probe. We usually perform the procedure in our outpatient office with high-resolution U/S machines, using a high-frequency curvilinear transducer.