Smallness at Birth and Neonatal Death: Reexamining the Current Indicator Using Sibling Data.

Objective: The similarity in size among siblings has implications for neonatal death, but research in this area is lacking in the United States. We examined the association between small-for-gestational age (SGA), defined as a birthweight <10th percentile for gestational age, and neonatal death, defined as death within the first 28 days of life, among second births who had an elder sibling with SGA ("repeaters") versus those whose elder sibling did not have SGA ("nonrepeaters").

Study Design: We conducted a population-based retrospective cohort study including 179,436 women who had their first two nonanomalous singleton live births in Missouri (1989-2005). Logistic regression was used to evaluate the association between SGA and neonatal death among second births, stratified by whether the elder sibling was SGA.

Results: Out of 179,436 second births, 297 died in the neonatal period. There was a significant interaction between birthweight-for-gestational age of first and second births in relation to neonatal death ( = 0.001). Second births with SGA had increased odds of neonatal death by 2.15-fold if they were "repeaters," and 4.44-fold if they were "nonrepeaters," as compared with non-SGA second births.

Conclusion: Our findings suggest that referencing sibling birthweight may be warranted when evaluating infant size in relation to neonatal death.