Amniocentesis is generally performed between 15-20 weeks gestation with a cytogenetic accuracy rate of over 99%. Miscarriage rates are about 1:250. There may be a 1:1000 risk for vaginal bleeding, amniotic fluid loss, and chorioamnionitis. Fetal trauma and culture failure are minimal. There are publications that have shown mid-trimester amniocentesis to have variable risk percentages. This range of miscarriage risk can vary from 0.5% to possibly as low as 0.06%. Third trimester amniocentesis may have a complication rate of 3% that would include preterm labor, rupture of membranes, bleeding, however, intrauterine fetal death and emergency delivery was not noted in one study. With amniocentesis, chromosomal analysis, amniotic fluid alpha-fetoprotein analysis and acetyl cholinesterase studies can be performed. During the procedure, asepsis should be observed at all times. If multiple gestation is present, use of indigo carmine to differentiate gestational sacs instead of methylene blue, should occur. Methylene blue may cause fetal staining, hemolytic anemia and methemoglobinemia resulting in intrauterine fetal death. Ultrasound must be performed beforehand to evaluate fetal number, viability, gestational age, placental location and amniotic fluid volume. Amniocentesis should be performed under continuous ultrasound guidance. Initial samples should be discarded in order to decrease the chance of maternal contamination. Every laboratory has a volume of amniotic fluid that is desired; in general 20 cc's is usually adequate. If the mother is Rh negative, Rhogam 300 meg should be given to the patient. Fetal cells are cultured and analyzed, and fetal heart rate post-amniocentesis should be performed.
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