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Summary Acetylcholinesterase (AChE) activity in amniotic fluid was assayed directly by a reaction rate method at 30°C using acetylthiocholine iodide as substrate and ethopropazine as a safe ‘pseudo’ cholinesterase inhibitor. Fresh samples from 101 normal pregnancies of 14–24 weeks gestation had a mean AChE activity of 2.56 u/1 (SD 1.10). Elevated levels of AChE were found in association with open spina bifida (5.5–20.4 u/1), anencephaly (10‐2‐19‐5 u/1), exomphalos (2.7–15.6 u/1), intrauterine death (30.2–59.3 u/1) and Turner's syndrome (36.4 u/1) but not with closed spina bifida or rhesus isoimmunisation. When AChE activity was expressed as a percentage of the total cholinesterase activity (‘percentage AChE’), there was a good correlation between AChE activity and‘percentage AChE’in normal pregnancies and the values associated with the 37 pregnancies affected by open neural tube defects (NTD) fell outside the 99.9 per cent confidence limits of the normal group. Qualitative differences in cholinesterase activity could be demonstrated between the groups with exomphalos and open NTD. It is suggested that the assay might be used satisfactorily to demonstrate the presence of open NTD in affected pregnancies.

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