title: Maternal prepregnancy body mass index and risk of spontaneous preterm birth.
abstract: BACKGROUND
  Findings from studies examining risk of preterm birth associated with elevated prepregnancy body mass index (BMI) have been inconsistent.
  
  
  METHODS
  Within a large population-based cohort, we explored associations between prepregnancy BMI and spontaneous preterm birth across a spectrum of BMI, gestational age, and racial/ethnic categories. We analysed data for 989,687 singleton births in California, 2007-09. Preterm birth was grouped as 20-23, 24-27, 28-31, or 32-36 weeks gestation (compared with 37-41 weeks). BMI was categorised as <18.5 (underweight); 18.5-24.9 (normal); 25.0-29.9 (overweight); 30.0-34.9 (obese I); 35.0-39.9 (obese II); and ≥ 40.0 (obese III). We assessed associations between BMI and spontaneous preterm birth of varying severity among non-Hispanic White, Hispanic, and non-Hispanic Black women.
  
  
  RESULTS
  Analyses of mothers without hypertension and diabetes, adjusted for age, education, height, and prenatal care initiation, showed obesity categories I-III to be associated with increased risk of spontaneous preterm birth at 20-23 and 24-27 weeks among those of parity 1 in each race/ethnic group. Relative risks for obese III and preterm birth at 20-23 weeks were 6.29 [95% confidence interval (CI) 3.06, 12.9], 4.34 [95% CI 2.30, 8.16], and 4.45 [95% CI 2.53, 7.82] for non-Hispanic Whites, non-Hispanic Blacks, and Hispanics, respectively. A similar, but lower risk, pattern was observed for women of parity ≥ 2 and preterm birth at 20-23 weeks. Underweight was associated with modest risks for preterm birth at ≥ 24 weeks among women in each racial/ethnic group regardless of parity.
  
  
  CONCLUSIONS
  The association between women's prepregnancy BMI and risk of spontaneous preterm birth is complex and is influenced by race/ethnicity, gestational age, and parity.
authors.affiliation: Department of Pediatrics, Stanford University School of Medicine, Palo Alto, CA.
authors.forename: Gary M
authors.initials: GM
authors.lastname: Shaw
authors.forename: Paul H
authors.initials: PH
authors.lastname: Wise
authors.forename: Jonathan
authors.initials: J
authors.lastname: Mayo
authors.forename: Suzan L
authors.initials: SL
authors.lastname: Carmichael
authors.forename: Catherine
authors.initials: C
authors.lastname: Ley
authors.forename: Deirdre J
authors.initials: DJ
authors.lastname: Lyell
authors.forename: Bat Zion
authors.initials: BZ
authors.lastname: Shachar
authors.forename: Kathryn
authors.initials: K
authors.lastname: Melsop
authors.forename: Ciaran S
authors.initials: CS
authors.lastname: Phibbs
authors.forename: David K
authors.initials: DK
authors.lastname: Stevenson
authors.forename: Julie
authors.initials: J
authors.lastname: Parsonnet
authors.forename: Jeffrey B
authors.initials: JB
authors.lastname: Gould
mesh_terms: D000328:Adult; D001741:Black or African American / Q000706:statistics & numerical data; D015992:Body Mass Index; D002140:California; D015331:Cohort Studies; D005260:Female; D005865:Gestational Age; D006630:Hispanic or Latino / Q000706:statistics & numerical data; D006801:Humans; D009765:Obesity / Q000150:complications*; D007752:Obstetric Labor, Premature / Q000453:epidemiology* / Q000209:etiology; D010298:Parity; D011247:Pregnancy; D011248:Pregnancy Complications / Q000453:epidemiology / Q000209:etiology; D047928:Premature Birth / Q000453:epidemiology* / Q000208:ethnology / Q000209:etiology; D044465:White People / Q000706:statistics & numerical data
keywords: low weight; obesity; pregnancy; race disparity