False-positive acetylcholinesterase with early amniocentesis. 
Among 93 acetylcholinesterase determinations performed on amniotic fluid samples from pregnancies at 11-14 weeks' gestation, five unexplained false-positive results were observed.  In four of the five cases, the ratio of acetylcholinesterase to pseudocholinesterase was compatible with that observed in association with open neural tube defects in later gestation.  In contrast, no false-positive results were noted among 951 acetylcholinesterase determinations performed on samples from women at 15-20 weeks' gestation.  Repeat amniocentesis was performed several weeks after the first procedure in four of the five cases of early amniocentesis and false-positive results; in each case, the acetylcholinesterase was negative on the second sample.  All four pregnancies had a normal outcome.  In the remaining case, trisomy 21 was diagnosed in the fetus and the pregnancy was terminated.  Positive acetylcholinesterase results should be interpreted cautiously in samples from early amniocentesis, especially when the amniotic fluid alpha-fetoprotein level is not markedly elevated.  The acetylcholinesterase-to-pseudocholinesterase ratio is not useful in identifying fetal neural tube defects before 15 weeks' gestation.  Repeat amniocentesis may help in determining the significance of a positive acetylcholinesterase result from early amniocentesis when no fetal defect is identified by ultrasonography.