Population-based recurrence risk of sudden infant death syndrome compared with other infant and fetal deaths.

To investigate the recurrence of sudden infant death syndrome (SIDS) among siblings, the authors analyzed data for all 352,475 mothers whose first and second single births were reported to the Medical Birth Registry of Norway during 1967-1988. Recurrence of stillbirths from the 16th week of gestation onward and infant deaths other than SIDS were also studied. Relative risk of recurrence for SIDS was 5.8 (95% confidence interval (CI) 2.1-13.2); for asphyxia- and immaturity-related infant deaths, 12.5 (9.2-17.4); for congenital malformations, 7.2 (4.7-11.0); and for other causes of infant death, 8.0 (2.0-22.1). Deaths due to infections did not recur. Similar categories of infant deaths had higher overall relative risk, 9.1, compared with 1.6 for dissimilar categories. Previous early stillbirth (16-27 weeks) had a high recurrence (relative risk (RR) = 21.8, 95% CI 17.5-26.9), while late stillbirth (> or = 28 weeks) had lower recurrence (RR = 4.6, 95% CI 3.7-5.8). Previous SIDS was associated with an increased risk of all other types of loss. In contrast, previous late stillbirth and previous asphyxia- and immaturity-related infant deaths were associated with a reduced risk of subsequent SIDS (RR = 0.31, 95% CI 0.08-0.84, and RR = 0.23, 95% CI 0.01-1.13, respectively). In conclusion, as with other infant and fetal deaths, SIDS deaths showed strong sibship aggregation consistent with a genetic susceptibility in subsets of SIDS that may interact with environmental factors. The authors also suggest common pregnancy-specific risk factors for late stillbirths, asphyxia- and immaturity-related infant deaths, and SIDS.