Exploring the association between short/long preceding birth intervals and child mortality: using reference birth interval children of the same mother as comparison

Background: This study used data from recent Demographic and Health Surveys (DHS) to examine the impact of short or long preceding birth intervals on neonatal and under-five mortality. In order to minimize the effect of selection issues, we examined child mortality outcomes of the same mother, comparing short or long interval births against births with what had previously been considered optimal intervals. Methods: We analyzed 47 DHS datasets from low-and middle-income countries. For each dataset, we compared neonatal and under-five mortality of short preceding interval births (< 18 months, < 24 months) to reference interval births (24-< 60 months) of a mother, using conditional logistic regression matching on the mother. We also conducted the same matched analysis for long (>= 60 months, >= 72 months) preceding interval births. These associations were then meta-analyzed. We also stratified the analyses by mothers' completed fertility (fertility at end of reproductive period) to assess whether maternal characteristics highly correlated with completed fertility modify the association between birth interval and child mortality. Results: Children with shorter preceding intervals had increased odds of both neonatal (< 24 months, OR: 1.61, 95% CI: 1.52-1.70) and under-five mortality (< 24 months, OR: 1.48, 95% CI: 1.40-1.56). When the associations were stratified by the mothers' completed fertility, the impact of short intervals was greatly reduced or eliminated for low fertility mothers. In contrast, mortality associations became stronger for children of high fertility mothers. However, when the births of high fertility mothers were limited to birth orders 2-4, the associations were comparable to those of low fertility mothers. Longer preceding birth intervals had lower odds of mortality than reference intervals (i.e. under-5 mortality for >= 60 months, OR 0.59, 95% CI: 0.52-0.67). This effect was also mediated by mothers' completed fertility; there was a strong protective effect of longer birth intervals for the high fertility mothers but not for low fertility mothers. Conclusions: These analyses reproduced findings reported in previous literature that shorter birth intervals are associated with higher child mortality. However the negative impact of short birth intervals may only occur in high parity births. Reproductive health interventions that seek to lengthen birth intervals may have larger impact by targeting women with high parity. This finding is consistent with the concept of maternal depletion as the underlying cause of increased adverse child outcomes associated with shorter birth intervals.