Consanguinity and recurrence risk of birth defects: a population-based study.

Recurrence risks give insight into the causes of birth defects and are useful in genetic counseling. There are few population-based studies of recurrence of birth defects for subsequent sibs with consanguineous parents. The aim of this study was to estimate and compare the recurrence risk of birth defects for offspring of first cousins and nonconsanguineous parents. The study population consisted of all single births with a previous sib born in Norway between 1967 and 1995. Altogether 660,398 children had nonconsanguineous parents, and 3,583 had parents who were first cousins. For nonconsanguineous parents the risk of a birth defect for the subsequent sib was 15 per 1,000 births (95% confidence interval: 14.5-15.1) if the previous child did not have a birth defect and 33 (95% confidence interval: 30-37) if the previous child had a birth defect. For parents who were first cousins the risk of a birth defect for the subsequent sib was 36 per 1,000 (95% confidence interval: 30-42) if the previous child did not have a birth defect and 68 (95% confidence interval: 33-122) if the previous child had a birth defect. The risk of recurrence of birth defects is higher for subsequent sibs with first-cousin parents than for those with nonconsanguineous parents. This difference indicates the degree to which the increased homozygosity among offspring of consanguineous parents influences the risk of recurrence of birth defects.