Background About 8% of deaths under age 1 year in England and Wales are unexplained, the cause of death being recorded as sudden infant death syndrome (SIDS, 60%) or unascertained (40%). Typically, unexplained infant death (UID) occurs during sleep, perhaps triggered by unsafe sleep environments (prone/side position, soft bedding etc.). Other risk factors include preterm birth, which may increase vulnerability. Incidence of both preterm birth and (in an analysis of data from 2005) SIDS has been relatively high for Black Caribbean babies. We aimed to evaluate recent ethnic variation in UID, test the hypothesis that it might be mediated by preterm birth, and assess several other potential explanatory factors.

Methods For singleton live births at gestation 22+weeks in England and Wales during 2006–2012, the Office of National Statistics provided a file linking birth and death registrations with other routine data. Variables included ethnicity (11 groups) and six covariates: gestational age, area deprivation (IMD quintile), gender, mother’s age at delivery, mother’s country of birth (UK-born or not), parental status at birth registration (within marriage, jointly registered by both parents at same address, joint registration with different addresses, sole registration). Using Stata, we fitted a sequence of logistic regression models for UID, starting with ethnicity and adding factors in the order in which they reduced the likelihood ratio test statistic for ethnic variation.

Results There were 4.6 million births and 1559 UID events (0.34 per 1000). Risk varied by ethnicity (p<0.001), being low for Black African, White non-British, and all South Asian groups, but high for Black Caribbean and two Mixed Black groups (African-White and Caribbean-White). Unadjusted odds ratios relative to White British ranged from 0.38 (95% CI 0.24–0.60) for Indian to 1.73 (1.21–2.47) for Black Caribbean. All covariates were associated with UID risk. When jointly adjusted for two factors (parental registration status and mother’s country of birth), the ethnic variation was not statistically significant (p=0.6). Other factors, including gestational age, made very little difference. Results were robust to sensitivity analysis for potential sparse-data bias.

Conclusion This large population-based study found substantial ethnic disparity in risk of UID, which was not attributable to preterm birth, maternal age or area deprivation. Parental registration status and mother’s country of birth jointly accounted for the variation, suggesting mediation by cultural factors. Better understanding of infant care practices in low-risk ethnic groups might lead to more effective prevention of UID in the general population.