title: The association between parity and birthweight in a longitudinal consecutive pregnancy cohort.
abstract: 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.
authors: Epidemiology Branch, Division of Intramural Population Health Research, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD.
authors: Stefanie N
authors: SN
authors: Hinkle
authors: Paul S
authors: PS
authors: Albert
authors: Pauline
authors: P
authors: Mendola
authors: Lindsey A
authors: LA
authors: Sjaarda
authors: Edwina
authors: E
authors: Yeung
authors: Nansi S
authors: NS
authors: Boghossian
authors: S Katherine
authors: SK
authors: Laughon
mesh_terms: D000293:Adolescent; D000328:Adult; D001724:Birth Weight; D015992:Body Mass Index; D005260:Female; D005865:Gestational Age; D006801:Humans; D016014:Linear Models; D008137:Longitudinal Studies; D008297:Male; D008423:Maternal Age; D010298:Parity / Q000502:physiology; D011247:Pregnancy; D012307:Risk Factors; D015430:Weight Gain / Q000502:physiology*
keywords: birth order; birthweight; parity; pregnancy; siblings