al., 1968a). The over-all incidence of the three malformations was 8-12 per thousand total births, and the individual incidences 4 13, 3 54, and 0-45, respectively; in male births 3-78, 1-73, and 0-32, and in female births 4-48, 5A40, and 0-58.The spina bifida incidence is the highest reported for any series for a whole population; that for anencephaly is exceeded only in the Northem Ireland series of Stevenson and Wamock (1959). The incidence of congenital hydrocephalus is unusually low, probably because in this survey all instances secondary to spina bifida were classed as spina bifida, only cases of truly congenital hydrocephalus were included, and instances where the presence of hydrocephalus was in doubt, for example in certain macerated stillbirths, were excluded. The eastem part of the district, East Monmouth, Ebbw Vale, Rhymney, and Caerphilly had a somewhat higher incidence than the western part, the comparative figures in the two areas being 4-61 and 3-85, 4-42 and 3-01, 0-42 and 0-46 for spina bifida, anencephaly, and hydrocephalus, respectively. METHODSFor a detailed study of familial aggregation and epidemiological characteristics, cases ascertained for the whole period in four areas-6, 7, 8, and 9 (Rhondda, Aberdare, Caerphilly, and Merthyr)-were included, but only cases born in 1960-62 inclusive in the other 8 areas. This group selected for detailed study (hereinafter called the main group) numbered 551, including 278 cases of spina bifida, 247 of anencephaly, and 26 of hydrocephalus. Half of these patients, selected by random numbers for each area, were matched with 277 controls*; these controls were the nearest births of the same sex from the same town or group of villages as the index patient in the local authority's birth register.Of this main group of 551 patients, 1 spina bifida and 1 anencephalus were not regarded as index patients for the family study, as they were not found in the original search but ascertained only through a sib who was an index patient. It was possible to visit the homes of the parents of all but 34 cases; however, at least partial family histories were obtained from health visitors, family doctors, or relatives for all but 1 patient with spina bifida and 1 with anencephalus, leaving 276 index patients with spina bifida, 245 index patients with anencephaly, and 26 with hydrocephalus. The parents of all but 15 of the 277 controls were visited, and again family

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