Influence of environmental factors in higher risk of sudden infant death syndrome linked with parental mental illness.

CONTEXT Since national risk reduction campaigns have been conducted, sudden infant death syndrome (SIDS) has become increasingly concentrated among disadvantaged families, including those affected by mental illness. However, causal mechanisms specific to this group are poorly understood. OBJECTIVES To estimate relative risk and compare risk factor prevalence in infants with and without parental psychiatric inpatient history, and to explore effect modification after the 1992 Swedish risk reduction campaign. DESIGN National birth cohort. Parental psychiatric admissions, maternal prenatal smoking, obstetric and social risk factors, and cause-specific infant death were ascertained via linkage between national registers. SETTING The Swedish population, 1978 through 2004. PARTICIPANTS All singleton live births (N = 2.5 million). MAIN OUTCOME MEASURE Incidence of SIDS. RESULTS Risk of SIDS was higher with a history of parental inpatient care, especially if both parents were admitted with any mental illness (odds ratio, 6.8; 95% confidence interval, 4.7-10.0), or if the mother (6.5; 4.9-8.7) or both parents (9.5; 5.5-16.4) had an alcohol/drug disorder. A 2-fold higher risk was also seen if the mother or father was admitted with any psychiatric illness other than alcohol or other drug disorders. Elevated risk persisted even if the last maternal inpatient episode had occurred 5 or more years before the infant's birth. After the national campaign, risk factor prevalence (especially maternal antenatal smoking) remained high in this population, and relative risks therefore increased. During 1992 through 2004, smoking and individual social adversity measures jointly accounted for approximately half the excess risk linked with maternal psychiatric inpatient history, whereas the confounding effects of obstetric factors were minimal. CONCLUSIONS Tailored approaches are needed to ensure that standard safety advice is effectively communicated to these vulnerable families. In particular, mentally ill pregnant women should be encouraged and better supported to stop smoking. Families with 2 affected parents require particularly strong support. A clearer understanding is needed as to why high risk factor prevalence persists among these parents.