Amniocentesis

Amniocentesis is an invasive diagnostic procedure which involves the transabdominal needle aspiration of amniotic fluid. A 10 to 20 mL sample of the amniotic fluid is withdrawn for analysis. This test is useful in detecting chromosomal abnormalities such as Down syndrome and neural tube defects such as spina bifida, in determining fetal maturity, and in detecting hemolytic disease of the newborn due to Rh incompatibility. It can also be used for determining gender, although the test is not done with this purpose in mind unless there is question of an X-linked chromosomal abnormality. The timing of the procedure is based on the reason the procedure is being done. If it is to determine genetic abnormalities, it is usually performed after the fifteenth week of pregnancy, when the two layers of fetal membranes have fused sufficiently to allow safe withdrawal of the amniotic fluid sample. To determine fetal maturity, the procedure is more likely to be performed after the thirty sixth week of gestation.
The amniotic fluid is analyzed for the following: acetylcholinesterase, alphafetoprotein, bacteria, bilirubin, chromosomal karyotype, color, creatinine, glucose, lecithin-to-sphingomyelin (L/S) ratio, meconium, and phosphatidylglycerol (PG).
Hemolytic disease of the newborn is indicated by high levels of bilirubin in the fluid. Meconium staining indicates possible fetal distress. Fetal pulmonary immaturity is demonstrated by a low L/S ratio and the absence of PG. A decreased creatinine level may also indicate fetal immaturity. Neural tube defects are suspected with increased levels of alpha-fetoprotein and acetylcholinesterase.