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Maternal age, birth order, and race: differential effects on birthweight
  1. Geeta K Swamy1,
  2. Sharon Edwards2,
  3. Alan Gelfand3,
  4. Sherman A James4,
  5. Marie Lynn Miranda2
  1. 1Department of Obstetrics and Gynecology, Duke University Medical Center, Durham, North Carolina, USA
  2. 2Nicholas School of the Environment, Duke University, Durham, North Carolina, USA
  3. 3Department of Statistics, Duke University, Durham, North Carolina, USA
  4. 4Sanford Institute of Public Policy, Duke University, Durham, North Carolina, USA
  1. Correspondence to Dr Marie Lynn Miranda, Children's Environmental Health Initiative, Nicholas School of the Environment and Earth Sciences, Box 90328, Durham, NC 27708, USA; mmiranda{at}


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.

  • Birthweight
  • fetal
  • inequalities
  • maternal age
  • parity
  • pregnancy
  • race
  • reproductive outcome

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  • This work was presented at the 41st Annual Society for Epidemiologic Research Meeting, Chicago, Illinois, 24–27 June 2008.

  • Funding This research was supported by funding from the National Institutes of Health (5P2O-RR020782-O3) and the Environmental Protection Agency (RD-83329301-0).

  • Competing interests None declared.

  • Ethics approval This study was conducted with the approval of the Duke's Institutional Review Board #1081.

  • Provenance and peer review Not commissioned; not externally peer reviewed.