Seasonal variation in the frequency of anencephalus and spina bifida births in the United Kingdom.

There have been statistically significant seasonal variations of annencephalus and spina bifida in each country of the United Kingdom during 1964-79 with the exception of anencephalus births in Northern Ireland. The seasonal peaks, where discernible, are in phase. Embryos appear to develop anencephalus most commonly in May-June and spina bifida in July.