Abstract 599 Epidemiology Topic Plenary, Monday, 5/3

Objective: Length of the interval between pregnancies has been reported as a risk factor for adverse outcomes such as low birth weight, preterm delivery, and infant death. The aim of this study was to compare the effects of short interpregnancy intervals on cause-specific infant mortality rates in the U.S.

Method: Using a 50 % random sample of the U.S. linked infant birth and death data for the year 1990 (N = 1,823,401), we estimated Cox proportional hazard models for overall infant mortality, as well as, cause-specific infant mortality due to 11 causes of death: maternal complications, complications of placenta, hypoxia and asphyxia, prematurity, congenital anomalies, respiratory distress syndrome (RDS), pneumonia, infections, sudden infant death syndrome(SIDS), unintentional and intentional injuries. In addition to interval, the adjusted proportional hazard models included birth weight as well as maternal age, education, marital status, maternal race/ethnicity, birth order, prenatal care, tobacco and alcohol use.

Results: Overall, there was about a two-fold increase in the risk of infant mortality that was associated with intervals <6 months and a 39 % increase with intervals 6-12 months for all-causes combined [Crude hazard ratios (HR): 1.95 and 1.39 respectively, p-values <0.0001]. However, most of this increase was due to the effects of the infants' birth weight and sociodemographic characteristics of mothers with short intervals (Adjusted HR, 1.24 and 1.25 for <6 and 6-12 months intervals, p-values <0.0001). Analysis by cause of death showed substantial differences in the effects of short intervals on cause-specific risks of infant death. Short intervals were associated with higher risks of death due to pneumonia (Adjusted HR, 2.12, 1.85, p< 0.0001). In models that controlled for sociodemographic factors, but not birth weight, short intervals were associated with increased risks of death due to RDS (Adjusted HR, 1.73 for <6 months, p < 0.0001; and 1.39 for 6-12 months, p=0.02) and prematurity (Adjusted HR, 1.82 for intervals <6 months, p<0.0001 - no association for 6-12 months intervals). There was no association between short intervals and risk of death due to congenital anomalies, maternal complications, hypoxia/asphyxia, and complications of placenta. On the other hand, there were higher risks for several causes of infant death that are related to childcare practices. There were higher risks associated with both < 6 months and 6-12 months intervals for infant deaths due to intentional injuries (Adjusted HR, 3.37, 2.57 for <6 and 6-12 months intervals respectively, p values < 0.0001), unintentional injuries (Adjusted HR, 1.74, 1.36, p<0.0001), and SIDS (Adjusted HR, 1.55, 1.68, p< 0.0001).

Conclusion: Short interpregnancy intervals are associated with increased risk of infant mortality due to several causes of death and in particular, those causes that are related to childcare practices.