Heightened risk of preterm birth and growth restriction after a first-born son

Abstract

PURPOSE:In Scandinavia, delivery of a first-born son elevates the risk of preterm delivery and intrauterine growth restriction of the next-born infant. External validity of these results remains unclear. We test this hypothesis for preterm delivery and growth restriction using the linked California birth cohort file. We examined the hypothesis separately by race/ethnicity. METHODS:We retrieved data on 2,852,976 births to 1,426,488 mothers with at least two live births. Our within-mother tests applied Cox proportional hazards (preterm delivery, defined as <37 weeks gestation) and linear regression models (birthweight for gestational age percentiles). RESULTS:For non-Hispanic whites, Hispanics, Asians, and American Indian / Alaska Natives, analyses indicate heightened risk of preterm delivery and growth restriction following a first-born male. The race-specific hazard ratios for preterm delivery range from 1.07 to 1.18. Regression coefficients for birthweight-for-gestational-age percentile range from -0.73 to -1.49. The 95% confidence intervals for all these estimates do not contain the null. By contrast, we could not reject the null for non-Hispanic black mothers.CONCLUSION: Whereas California findings generally support those from Scandinavia, the null results among non-Hispanic black mothers suggest that we do not detect adverse outcomes following a first-born male in all racial/ethnic groups.

Sex of the first‐born and obstetric complications in the subsequent birth. A study of 2.3 million second births from Denmark, Finland, Norway, and Sweden

“…In Denmark a male first‐born has been associated with later increased risks of recurrent pregnancy loss, stillbirth, and reduced birthweight 1‐4 . In addition, a first male offspring has been associated with later risks of preterm birth in studies based on Danish, Swedish, and Californian medical birth records 1,5 . Among younger brothers, the presence of an older brother has been associated with subtle differences in anthropometrics in young adulthood, which may be indicative of an enduring influence 6 .…”