Mortality in Infants Affected by Preterm Birth and Severe Small-for-Gestational Age Birth Weight.

Background: Few researchers have evaluated neonatal mortality in the combined presence of preterm birth (PTB) and small-for-gestational age (SGA) birth weight. None differentiated between infants with and without anomalies, considered births starting at 23 weeks' gestation, or defined SGA at a more pathologic cutpoint less than the fifth percentile.

Methods: We completed a population-based cohort study within the province of Ontario, Canada, from 2002 to 2015. Included were 1 676 110 singleton hospital live births of 23 to 42 weeks' gestation. Modified Poisson regression compared rates and relative risks of neonatal mortality among those with (1) preterm birth at 23 to 36 weeks' gestation and concomitant severe small for gestational age (PTB-SGA), (2) PTB at 23 to 36 weeks' gestation without severe SGA, (3) term birth with severe SGA, and each relative to (4) neither. Relative risks were adjusted for maternal age and stratified by several demographic variables.

Results: Relative to a neonatal mortality rate of 0.6 per 1000 term infants without severe SGA, the rate was 2.8 per 1000 among term births with severe SGA (adjusted relative risk [aRR] 4.6; 95% confidence interval [CI] 4.0-5.4), 22.9 per 1000 for PTB without severe SGA (aRR 38.3; 95% CI 35.4-41.4) and 60.0 per 1000 for PTB-SGA (aRR 96.7; 95% CI 85.4-109.5). Stratification by demographic factors showed a persistence of this pattern of neonatal death. Restricting the sample to births at ≥24 weeks' gestation, or newborns without a congenital or chromosomal anomaly, also demonstrated the same pattern.

Conclusions: Methods to detect or prevent PTB or SGA should focus on PTB-SGA, which serves as a useful perinatal surveillance indicator.