The association between parity and birthweight in a longitudinal consecutive pregnancy cohort

Paediatr Perinat Epidemiol. 2014 Mar;28(2):106-15. doi: 10.1111/ppe.12099. Epub 2013 Dec 9.


Background: Nulliparity is associated with lower birthweight, but few studies have examined how within-mother changes in risk factors impact this association.

Methods: We used longitudinal electronic medical record data from a hospital-based cohort of consecutive singleton live births from 2002-2010 in Utah. To reduce bias from unobserved pregnancies, primary analyses were limited to 9484 women who entered nulliparous from 2002-2004, with 23,380 pregnancies up to parity 3. Unrestricted secondary analyses used 101,225 pregnancies from 45,212 women with pregnancies up to parity 7. We calculated gestational age and sex-specific birthweight z-scores with nulliparas as the reference. Using linear mixed models, we estimated birthweight z-score by parity adjusting for pregnancy-specific sociodemographics, smoking, alcohol, prepregnancy body mass index, gestational weight gain, and medical conditions.

Results: Compared with nulliparas', infants of primiparas were larger by 0.20 unadjusted z-score units [95% confidence interval (CI) 0.18, 0.22]; the adjusted increase was similar at 0.18 z-score units [95% CI 0.15, 0.20]. Birthweight continued to increase up to parity 3, but with a smaller difference (parity 3 vs. 0 β = 0.27 [95% CI 0.20, 0.34]). In the unrestricted secondary sample, there was significant departure in linearity from parity 1 to 7 (P < 0.001); birthweight increased only up to parity 4 (parity 4 vs. 0 β = 0.34 [95% CI 0.31, 0.37]).

Conclusions: The association between parity and birthweight was non-linear with the greatest increase observed between first- and second-born infants of the same mother. Adjustment for changes in weight or chronic diseases did not change the relationship between parity and birthweight.

Keywords: birth order; birthweight; parity; pregnancy; siblings.

Publication types

  • Comparative Study
  • Research Support, N.I.H., Intramural

MeSH terms

  • Adolescent
  • Adult
  • Birth Weight*
  • Body Mass Index
  • Female
  • Gestational Age
  • Humans
  • Linear Models
  • Longitudinal Studies
  • Male
  • Maternal Age
  • Parity* / physiology
  • Pregnancy
  • Risk Factors
  • Weight Gain / physiology*