Defects of the central nervous system in Finland. II. Birth order, outcome of previous pregnancies and family history

Seven hundred and ten cases with defects of the central nervous system (CNS) reported to the Finnish Register of Congenital Malformations in 1965‐73 were analysed with regard to possible etiological factors. A time‐area matched birth for every case, 259 cases of polydactyly, and official statistics for the entire country were used as controls. High birth order turned out to be a risk factor especially for the group of hydrocephaly, maternal age not taken into account. When this confounding factor had been ruled out, it was found that high birth order was associated with anencephaly and high maternal age with hydrocephaly. However, a combination of both these factors gave the most striking association in both groups of defects. Higher rates of abortions and stillbirths in previous pregnancies, previous live births with CNS defects or any kind of defects were significantly associated with the birth of children with CNS defects. The rate of twinning was found to be higher in the study group than in the general population, and in the great majority of twin births the CNS defective child was the later born partner. Parental profession did not seem to be associated with the outcome of pregnancy. The significance of the socio‐economic status and other confounding factors are discussed in connection with the concept of the marital status of mothers.