SIB RISKS OF ANENCEPHALY AND SPINA-BIFIDA IN BRITISH-COLUMBIA

The prevalence at birth of anencephaly or spina bifida cystica among sibs of individuals born in British Columbia with these defects was studied using routinely collected records of morbidity and mortality within the province. The population prevalence of these neural-tube defects in British Columbia is approximately 1.55 per thousand births, lower than in other areas where family studies have been carried out. A total of 1,028 affected individuals born in British Columbia during the period 1952-1970 was ascertained from health and vital documents collected on an ongoing basis by the Division of Vital Statistics of the British Columbia Ministry of Health. Information on the families of these cases was acquired using computer-linked groupings of British Columbia marriage, birth, and stillbirth records. The risk of affected individuals among the sibs was 2.4%, about 15 times the population prevalence at birth. The prevalence among subsequent sibs of the first affected individual in a family was 2.1%. There was no difference in risk when various factors that influence the population incidence of these conditions were considered. The risk of recurrence of either of these anomalies after two previously affected sibs was 4.8%, or double the risk after one affected sib. From these results and those of a recent New York survey of sib risks of neural-tube anomalies it would seem that the risk of anencephaly and spina bifida cystica to sibs of affected cases in North America is approximately half the risk in Great Britain, and it is this lower risk of 2% that should be used for genetic counseling in North America.