Health Articles

AMNIOCENTESIS

Amniocentesis for prenatal diagnostic testing is usually offered between 15 and 20 weeks of gestation. Under ultrasound guidance, a 20-22-gauge spinal needle is passed into the amniotic fluid. The initial aspirate of 1-2 mL of fluid should be discarded to decrease the chance of maternal cell contamination. A total of 20-30 mL of fluid is aspirated, and the needle is removed.
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Amniocentesis has greater than 99% cytogenetic diagnostic accuracy and a fetal loss rate of approximately 0.5%. The most common complications have been transient vaginal spotting or amniotic fluid leakage in approximately 12% of all cases. The incidence of chorioamnionitis is less than 1 in 1,000. Needle injuries to the fetus have been reported but are very rare. Culture failure is uncommon.

Amniocentesis with twins is possible in 95% of pregnancies. Amniotic fluid is aspirated from the first sac. Before the needle is removed, 2-3 mL of indigo carmine (methylene blue is not recommended) diluted 1:10 in bacteriostatic water is injected into the first sac. Amniocentesis is then performed on the second sac at a location that is selected after visualization of the separating membrane. Aspiration of clear fluid confirms that the second sac has been sampled.

Ninety percent of NTDs are associated with intracranial abnormalities or varying degrees of hydrocephaly. The value of amniotic fluid alpha-fetoprotein determination with an ultrasound evaluation has become increasingly controversial as ultrasound resolution has improved. Through amniotic fluid alpha-fetoprotein analysis, diagnosis of an NTD is possible in all except the 5% of fetuses whose spinal defect is covered by skin. The amniotic fluid alpha-fetoprotein level may be spuriously elevated if the amniotic fluid is contaminated with fetal blood. This potential error can be eliminated, however, if amniotic fluid acetylcholinesterase is assayed simultaneously. Its presence verifies that the elevated amniotic fluid alpha-fetoprotein level is likely due to an NTD or other fetal defect. If fetal hemoglobin is present but acetylcholinesterase is absent, the elevated amniotic fluid alpha-fetoprotein level can be deduced to result from either the presence of fetal blood or from an anomaly other than an NTD. Failure to detect an anomaly by ultrasonography does not necessarily indicate that the elevated maternal serum alpha-fetoprotein level was spurious. A subtle anomaly may still exist. Alternatively, an unexplained elevated maternal serum alpha-fetoprotein level may reflect placental dysfunction and indicate increased risk for certain pregnancy complications (eg, preterm labor, IUGR, stillbirth).

This entry was posted on Tuesday, April 28th, 2009 at 4:10 pm and is filed under Amniocentesis. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.