Association Between Income and Perinatal Mortality in the Netherlands Across Gestational Age

While the contribution of socioeconomic status to disparities in neonatal death and stillbirth has received considerable attention, several key aspects remain understudied. These include the specificity of the association between household income and perinatal outcomes and the contribution of gestational age and birth weight to income-related disparities in perinatal mortality. This large nationally representative cross-sectional study aimed to examine the association between household-level income and perinatal mortality in the Netherlands while investigating the effect that birth weight mediates this association. Data on all registered singleton births born between 24 and 42 weeks of gestation in the Netherlands and household-level tax records were compiled between January 2004 and December 2016. Household income rank was the main exposure variable, calculated by ranking annual disposable income of the mother's household, adjusted by size, from 1 to 100 relative to the distribution of households with a childbirth for each year separately. The primary study outcome was perinatal mortality defined as intrauterine death occurring after 24 weeks of gestation or death up to 7 days after live birth. The association between household income rank and perinatal mortality was controlled for potential confounders including maternal age at delivery, parity, ethnicity, year of birth, and newborn sex. Birth weight centile adjusted for gestational age, and sex was included as a potential mediator of this association. A total of 2,036,431 births were included in this analysis, of which 121,010 (5.9%) were born preterm, and 8720 died during the perinatal period. Mothers of non-Western race/ethnicity comprised 40.6% of the lowest-income quintile and 7.0% of the highest-income quintile. Mean perinatal mortality was 4.3 per 1000 and decreased with increasing income rank from 5.9 in the lowest to 3.2 per 1000 in the highest. In unadjusted and when adjusting for confounders, higher household income rank was associated with lower perinatal mortality. When adjusting for confounding factors, the negative association between perinatal mortality and household income rank had a bottom-to-top ratio of 1.79 (95% CI, 1.62-1.98), which was attenuated to 1.30 (95% CI, 1.22-1.39) when adjusting for birth weight centile. When stratified by gestational age, there was a negative association on adjusted analysis between household income rank and perinatal mortality in all but the lowest gestational age category (24 weeks to 25 weeks 6 days) with bottom-to-top ratios ranging from 1.21 (95% CI, 1.05-1.43) between 26 and 28 weeks and 1.29 (95% CI, 1.13-1.51) between 28 and 32 weeks. Sensitivity analysis confirmed a negative association between household income and perinatal mortality other than among early gestational ages. This large cross-sectional study demonstrated an association between maternal household income rank and perinatal mortality that was only partially mediated when controlling for birth weight and was absent among infants born at less than 26 weeks of gestation. In addition, each increase in household income rank carried a larger decrease in mortality at the bottom half of the income distribution than in the top half, suggesting income differences at the bottom end are more impactful.