Background
Studies examining the influence of maternal age and birth order on
birthweight have not effectively disentangled the relative contributions of
each factor to birthweight, especially as they may differ by race.
Methods
A population-based, cross-sectional study of North Carolina births
from 1999 to 2003 was performed. Analysis was restricted to 510 288
singleton births from 28 to 42 weeks’ gestation with no congenital
anomalies. Multivariable linear regression was used to model maternal age
and birth order on birthweight, adjusting for infant sex, education, marital
status, tobacco use and race.
Results
Mean birthweight was lower for non-Hispanic black individuals (NHB,
3166 g) compared with non-Hispanic white individuals (NHW, 3409 g) and
Hispanic individuals (3348 g). Controlling for covariates, birthweight
increased with maternal age until the early 30s. Race-specific modelling
showed that the upper extremes of maternal age had a significant depressive
effect on birthweight for NHW and NHB (35+ years, p<0.001), but only
age less than 25 years was a significant contributor to lower birthweights
for Hispanic individuals, p<0.0001. Among all racial subgroups,
birth order had a greater influence on birthweight than maternal age, with
the largest incremental increase from first to second births. Among NHB,
birth order accounted for a smaller increment in birthweight than for NHW
and Hispanic women.
Conclusion
Birth order exerts a greater influence on birthweight than maternal
age, with signficantly different effects across racial subgroups.
