Birth Order and Injury-Related Infant Mortality in the U.S.

Abstract

Introduction The purpose of this study was to evaluate the risk of death during the first year of life due to injury, such as unintentional injury and homicide, by birth order in the U.S. Methods Using national birth cohort–linked birth–infant death data (births, 2000–2010; deaths, 2000–2011), risks of infant mortality due to injury in second-, third-, fourth-, and fifth or later–born singleton infants were compared with first-born singleton infants. Risk ratios were estimated using log-binomial models adjusted for maternal age, marital status, race/ethnicity, and education. The statistical analyses were conducted in 2016. Results Approximately 40%, 32%, 16%, 7%, and 4% of singleton live births were first, second, third, fourth, and fifth or later born, respectively. From 2000 to 2011, a total of 15,866 infants died as a result of injury (approximately 1,442 deaths per year). Compared with first-born infants (2.9 deaths per 10,000 live births), second or later–born infants were at increased risk of infant mortality due to injury (second, 3.6 deaths; third, 4.2 deaths; fourth, 4.8 deaths; fifth or later, 6.4 deaths). The corresponding adjusted risk ratios were as follows: second, 1.84 (95% CI=1.76, 1.91); third, 2.42 (95% CI=2.30, 2.54); fourth, 2.96 (95% CI=2.77, 3.16); and fifth or later, 4.26 (95% CI=3.96, 4.57). Conclusions Singleton infants born second or later were at increased risk of mortality due to injury during their first year of life in the U.S. This study’s findings highlight the importance of investigating underlying mechanisms behind this increased risk.

Good practices for the design, analysis, and interpretation of observational studies on birth spacing and perinatal health outcomes

“…4 Likewise, one could speculate that the impact of a short interpregnancy interval on child injury may differ if a family has two other young children at home competing for attention, rather than only one. 53,54 Although it is difficult to prove the validity of this assumption, researchers can explore its plausibility in several ways. First, they can present results stratified by birth order to confirm that estimates remain similar irrespective of whether the short interpregnancy interval was a woman's first vs second interval.…”

Beyond birth outcomes: Interpregnancy interval and injury‐related infant mortality

“…Graphs of Schoenfeld residuals showed uniform distri- provides a reasonable approximation of risk even in the presence of competing risks. 6,7,31 Confounders were selected based on the literature and whether the variable was associated (and preceded) the interpregnancy interval and outcome. 6,7,16,25,32 Our final models were adjusted for:…”

“…Additionally, cumulative and instantaneous hazard of external cause of death curves by IPI categories were generated using the Kaplan‐Meier survival function estimator. In situations such as ours, where the risk of outcome was rare, this estimator provides a reasonable approximation of risk even in the presence of competing risks …”

“…Underlying cause of death was coded using the International Classification of Disease, tenth revision (ICD‐10), and used to determine external causes of infant deaths (ie injury‐related) from birth to 365 days: accidents (unintentional injuries; V01‐X59); assault (homicide; *U01, X85‐Y09); complications of medical and surgical care (Y40‐Y84); and other external causes (Y10‐Y36). Accidental suffocation and strangulation in bed (W75) is the most common external cause of death code among infants and one of three codes used to classify sudden unexpected infant death (SUID) (W75, R95, and R99). Due to diagnostic shifts in classification of sleep‐related deaths over time from R95 to W75, we examined the following alternative groupings of injury‐related ICD‐10 codes: external causes excluding W75, external causes plus two additional causes of death used to classify SUID (SIDS [R95] and other ill‐defined and unspecified causes of mortality [R99]), and SUID alone (W75, R95, and R99).…”

“…Higher birth order and plurality have been associated with an increased risk of external causes of infant mortality, consistent with the hypothesis that divided parental time and resources combined with increased childcare responsibilities may play a role in infant injury risk. 6,7 A related factor, closely spaced births, may also be associated with external causes of infant death, but has not been examined for the United States, in part because national data have not been available since the early 1990s.…”