1 Department of Epidemiology, Harvard School of Public Health
An analysis of 776 cases of anencephaly and spina bifida, born to Jewish mothers in Israel between 1958 and 1968, is presented. A prevalence of 0.86 per 1,000 total births for anencephaly and 0.60 for spina bifida was observed. A high sex ratio for anencephaly (44.1% male) was observed and it is suggested that high ratios may be characteristic of some populations with a low prevalence of neural tube defects. High rates (2.1 per 1,000 for both anomalies) were observed among children of mothers who emigrated from Iraq and Iran. Lowest rates were observed among Israeli mothers. Prevalence rates had their strongest associations with birth order, from a low of 1.25 per 1,000 among first births to a high of 2.40 per 1,000 for the sixth and higher birth orders. The effect of birth order was independent of maternal age and ethnic variation. No significant seasonal variation of affected births was detected. But a very strong downward trend was observed during the study period, mainly in spina bifida (from a prevalence of 1.05 per 1,000 in 1958 to 0.44 in 1968). The trend could not be explained by changes in parity or ethnic composition of the population and an environmental hypothesis is proposed.
Submitted on May 12, 1970