Most congenital anomalies are believed to result from interactions between genetic and environmental determinants, whose relative importance is not generally established. Temporal and spatial patterns allow inferences on the underlying transmission processes; in particular, it is possible to discriminate between sporadic and nonsporadic genetic factors, and to find evidence for the effects of environmental heterogeneity in time and space. We studied the occurrence of 14 anomalies in 14 registries of Western Europe Four basic patterns have been identified:
(1) Chromosomal abnormalities have uniform incidences and do not show significant geographical variation, in agreement with the expected consequences of randomly scattered nondisjunction events. ( 2 ) The homogeneous spatial distributions of three severe malformations (renal agenesis, oesophageal atresia, ano-rectal atresia) are consistent with both the effects of fresh mutation and segregation of detrimental alleles. ( 3 ) A decrease of similarity of incidences with distance has been observed for neural tube defects, and ithis is the expected consequence of isolation by distance on genetically determined traits. (4) For facial clefts, polydactyly , and hypospadias, all postulated processes poorly account for the observed temporal and spatial patterns.