As Carter points out, anencephaly and spina bifida, the most common of the neural tube malformations, are in terms of public health the most important congenital malformations in Britain, and especially in the north and west of Britain. It is a general finding that a child with spina bifida or anencephaly is more likely to have a sib with either type of malformation, suggesting that all neural tube malformations spring from similar causes. The incidence of neural tube malformations also shows consistent trends for years at a time. The cultural and geographical environments of these populations are so different that one must assume their relative immunity is due to a genetic factor. There are a number of clinical and ethical problems associated with this screening procedure. Studies using this method of screening with a large sample of pregnant women indicate that it is possible to identify over 90 per cent of foetuses with anencephaly or open myelomeningocele.