School of Public Health

Jennifer Maas

Family size, birth interval, and the risk of serious unintentional injury to the youngest child


To evaluate the association between family size/spacing and risk of fatal and serious non-fatal unintentional injury to children younger than 15 months of age. We also sought to determine whether the magnitude of this association depends on the injury mechanism.


A population-based case control study was conducted using linked hospital discharge and vital records data from Washington State. All children born in Washington State who were subsequently hospitalized (n=3838) or died (N=342) at <15 months of age as a result of an unintentional injury during 1991-2012 were identified in the hospital discharge and death certificate data. Injuries were classified based on ICD-9 and ICD-10 codes. Controls (n=65,711) were randomly selected from Washington State birth certificates of those without an injury-related hospitalization or death during the same time period. Mantel-Haenszel stratified analysis was conducted using data from birth certificates of cases and controls.


Larger family size was associated with increased risk of serious pediatric injury (odds ratio (OR) 1.80 for parity 3+, 95% confidence interval (CI) 1.61-2.11 compared with parity=0), with a dose-response relationship observed. The magnitude of the increased risk depended on the injury mechanism, with the largest risk for injuries involving drowning/suffocation/foreign body (OR 2.93 for parity 3+, 95% CI 2.33-3.68 compared with parity=0) and being struck by/against an object (OR 3.47 for parity 3+, 95% CI 1.64-7.30 compared with parity=0). Children born <15 months after a sibling had an increased risk of serious injury (OR 1.63, 95% CI 1.38-1.94) compared to those whose sibling was >30 months older.


The results indicate an association between higher parity and increased risk of serious injury to the youngest child, with magnitude of risk depending on injury mechanism. In addition, very close birth interval between the two youngest children is associated with increased risk of serious injury to the younger child. These results suggest priorities for targeting injury prevention interventions.