Second trimester amniotic fluid acetylcholinesterase quantitation in the prenatal diagnosis of neural tube defects.

Amniotic fluid acetylcholinesterase (AChe) activity was assayed in second trimester amniotic fluids by inhibition of non-specific cholinesterase using lysivane. Of the 196 samples analysed, 146 were from normal pregnancies; 11 (5.6%) from severe open neural tube defects; two from small open spina bifidas; seven from fetal chromosome abnormalities and two from severe skeletal dysplasias. In addition nine amniotic fluids were meconium stained and 19 (9.7%) had varying degrees of blood staining. Acetylcholinesterase values remained relatively constant from 13-21 weeks gestation in normal pregnancies and a cut-off limit of 3.2 U/l was set (mean + 2 SD). Heavily blood stained amniotic fluids were associated with AChe levels on or above the normal cut-off limit. Meconium staining caused a non-significant elevation of AChe. No significant elevation of AChe was demonstrated for fetal chromosome abnormalities, or severe skeletal dysplasias. Anencephaly and severe open spina bifida had significantly elevated AChe levels.