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

Area of Science:

• Obstetrics and Gynecology
• Perinatal Epidemiology
• Reproductive Health

Background:

• Previous Scandinavian studies indicated that a first-born son elevates risks for subsequent preterm delivery and intrauterine growth restriction.
• The external validity of these findings across different populations remained uncertain.

Purpose of the Study:

• To investigate the association between a first-born son and subsequent risks of preterm delivery and intrauterine growth restriction.
• To examine these associations separately across various racial and ethnic groups.

Main Methods:

• Utilized the linked California birth cohort file, analyzing data from 2,852,976 births to 1,426,488 mothers with at least two live births.
• Employed within-mother Cox proportional hazards models for preterm delivery (less than 37 weeks gestation) and linear regression models for birth weight for gestational age percentiles.

Main Results:

• A first-born male was associated with increased risks of preterm delivery and growth restriction for non-Hispanic whites, Hispanics, Asians, and American Indian/Alaska Natives (Hazard Ratios for preterm delivery: 1.07-1.18; Regression coefficients for birth weight percentile: -0.73 to -1.49).
• These associations were statistically significant across these groups, with confidence intervals not including the null value.
• No statistically significant association was found between a first-born male and adverse outcomes for non-Hispanic Black mothers.

Conclusions:

• Findings from California generally support Scandinavian research on the adverse effects of a first-born son on subsequent pregnancies.
• However, the absence of detected adverse outcomes in non-Hispanic Black mothers suggests that this association may not be universal across all racial and ethnic groups.