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Forty-two open neural tube defects (NTDs) were identified in our series of 7440 amniocenteses tested between 11 and 15 weeks of gestation. Using a cut-off of > or = 2.0 MOM, the detection rate for open NTDs was 95 per cent; 100 per cent each for anencephaly and spina bifida; and 78 per cent for encephalocele. Two encephaloceles had AFP levels less than 2.0 MOM and negative AChEs. Thirty-four (81 per cent) of these NTDs were tested between 13 and 15 weeks and 8 (19 per cent) before 13 weeks. There were 0.6 per cent false positives by AFP (excluding serious abnormalities and fetal death) and 0.1 per cent after AChE. The likelihood of an open NTD after an elevated AFP (> or = 2.0 MOM) was 24 and 77 per cent for any serious abnormality. These results, when combined with an earlier study, indicate that amniotic fluid AFP appears to be as sensitive a test for open NTDs between 13 and 15 weeks as between 16 and 20 weeks. Additional experience is necessary to determine this before 13 weeks.

In the 2 years between 1993 and 1995, we assayed alpha‐fetoprotein (AFP) in 48 412 amniotic fluids (AFs). One thousand and eighty‐six (2.2 per cent) measured ≥2.0 MOM and these were subdivided into three groups; mildly (2.0–4.9 MOM), moderately (5.0–9.9 MOM), and very elevated (≥10.0 MOM). Abnormalities occurred in 25 per cent of the mildly elevated compared with 88 per cent of the moderately and 98 per cent of the very elevated cases. Forty‐five per cent of the neural tube defects (NTDs) had AF‐AFPs in the 5.0–9.9 and 36 per cent in the ≥10.0 MOM range. After a positive acetylcholinesterase (AChE) test, both the moderately and the very elevated groups had abnormalities in ≥95 per cent of cases, compared with 85 per cent in the mildly elevated group. After a negative AChE test, abnormalities occurred in 79, 52 and 18 per cent in the very elevated, moderately, and mildly elevated groups, respectively. Excluding chromosome abnormalities, an abnormal twin, and bloody samples, the risk of a fetal abnormality after a normal ultrasound was less than 1 per cent if the AChE was positive in both the moderately and the very elevated groups. If the AChE was negative, the risk was 18 per cent for the moderately and 55 per cent for the very elevated groups, of which congenital nephrosis accounted for 75 per cent; AFP levels usually increased in a later AF sample. Repeat amniocentesis may be offered to women with AF‐AFPs ≥5.0 MOM if no abnormality is seen with high‐resolution ultrasound. © 1997 John Wiley & Sons, Ltd.

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