The incidence of anencephalus in the Fylde peninsula 1956-76 and changes in water hardness.

In an area which had a high incidence of anencephalus, 3.2 per 1000 births, there was a significant drop to 1.3 per 1000, below the national average of 1.7 to 1.8, among conceptions after 1967. In the northern part of the area this drop was greater in summer than winter conceptions, providing additional evidence of a different process underlying case occurrences in the north of the area from that in the south. In 1957-61 there were significantly more births of anencephalic babies in North Fylde than in South Fylde, but both areas were supplied with soft water. From 1962 to 1969 the water changed from soft to slightly hard. Soft water does not appear to be a primary aetiological factor in anencephalus, but hard water may mitigate the effect of other factors.