Background: Certain birth defects are known to recur in families, but most estimates of the risk of recurrence have come from clinic-based studies. In this study we estimated the risk of recurrent birth defects using a population-based registry.
Methods: The study was based on the records of the first and second infants delivered by 371,933 women from 1967 through 1989. The records are maintained by the Medical Birth Registry of Norway. For the 9192 women whose first infant had a birth defect, we determined the relative risk of similar and dissimilar defects in the second infant. The reference population was women whose first infant had no defect.
Results: Among first infants, 2.5 percent had a birth defect. The mothers of affected first infants were 2.4 times as likely as other women to have second infants with any registered defect. This increased risk was due primarily to an increased (7.6 times higher) risk of the same defect in the second infant as in the first (95 percent confidence interval, 6.5 to 8.8) and secondarily to a slightly increased (1.5 times higher) risk of a different defect in the second infant (95 percent confidence interval, 1.3 to 1.7). Among the women who lived in the same municipality during both pregnancies, the relative risk of having a second infant with the same defect was 11.6, as compared with 5.1 among the women who moved to another municipality after the birth of their first infant (P < 0.001).
Conclusions: Among women whose first infant has a birth defect, the risk of the same defect in the second infant is substantially increased and the risk of a different defect in the second infant is slightly increased. Environment plays a strong part in repeated defects.