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

Using data from the Northern Congenital Abnormality Survey, we describe tre nds 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 po pulation-based sample consisted of 934 NTDs: there were 403 (43.1%) with an encephaly, 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 ove r time (chi(2) for trend = 7.2; P = 0.007). The overall birth prevalence wa s 5.6 per 10 000 births [95% CI = 5.0, 6.3]; there was also a significant r eduction in birth prevalence with time (chi(2) for trend = 68.3; P < 0.0001 ). Maternal age-specific prevalence rates decreased with increasing age. Th e 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 < 0.0 01). The sensitivity of antenatal diagnosis was consistently high for anenc ephaly (98%) and increased significantly for spina bifida from 60% during 1 984-90 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 bet ween 1984-90 and 1991-96. This has resulted from improvements in the accura cy of antenatal detection of NTD-affected pregnancies with an increase in t erminations of pregnancy.