The changing prevalence of neural tube defects: a population‐based study in the North of England, 1984–96

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Abstract
Using data from the Northern Congenital Abnormality Survey, we describe trends in the prevalence of neural tube defects (NTDs), maternal age‐specific prevalence, outcome of pregnancy and the sensitivity of antenatal diagnosis of NTD‐affected pregnancies in the Northern Region during 1984–96. This population‐based sample consisted of 934 NTDs: there were 403 (43.1%) with anencephaly, 472 (50.5%) with spina bifida and 59 (6.3%) with encephalocele. The total prevalence at birth was 17.9 per 10 000 births and terminations (95% confidence interval [CI] = 16.7, 19.0) with a significant reduction over time (χ2 for trend = 7.2; P = 0.007). The overall birth prevalence was 5.6 per 10 000 births [95% CI = 5.0, 6.3]; there was also a significant reduction in birth prevalence with time (χ2 for trend = 68.3; P < 0.0001). Maternal age‐specific prevalence rates decreased with increasing age. The proportion of NTD pregnancies terminated increased from 60.3% (325 cases) during 1984–90 to 78.6% (293 cases) during 1991–96, whereas the proportion of livebirths declined from 31.7% (171 cases) to 15.0% (56 cases) (P < 0.001). The sensitivity of antenatal diagnosis was consistently high for anencephaly (98%) and increased significantly for spina bifida from 60% during 1984–90 to 85% during 1991–96 (P < 0.05). Ascertainment of all cases of NTD in the Northern Region revealed a twofold reduction in birth prevalence between 1984–90 and 1991–96. This has resulted from improvements in the accuracy of antenatal detection of NTD‐affected pregnancies with an increase in terminations of pregnancy.