Maternal prior pregnancy loss and the sex ratio among infants with sudden infant death syndrome.

It has been reported that the offspring of human leukocyte antigen (HLA)-compatible parents may have a high male/female sex ratio. To study the hypothesis that parental HLA compatibility is related to sudden infant death syndrome (SIDS), I conducted a population-based case-control study using 1982-1990 linked Washington State birth and death certificate data to compare the sex ratio of SIDS cases and controls. SIDS cases were identified by the International Classification of Diseases, 9th revision, CM code 798.0, on the death certificate, and four controls born in the same year as the case were randomly selected for each case. I categorized the infants according to maternal parity and history of pregnancy loss. There was a high risk of SIDS in males relative to females among first liveborn infants whose mothers had had two or more prior pregnancy losses (odds ratio = 7.6, 95% confidence interval = 1.5-39). I had hypothesized a priori that this group would have the largest proportion of infants of HLA-compatible parents. No similar association was observed among infants with a prior liveborn sibling. In addition to the findings from two previous studies, this finding provides further evidence that parental HLA compatibility may be related to the risk of SIDS in offspring.