title: Women's prepregnancy underweight as a risk factor for preterm birth: a retrospective study.
abstract: To investigate the distribution of known factors for preterm birth (PTB) by severity of maternal underweight; to investigate the risk-adjusted relation between severity of underweight and PTB, and to assess whether the relation differed by gestational age.
  
  
  
  Retrospective cohort study.
  
  
  
  State of California, USA.
  
  
  
  Maternally linked hospital and birth certificate records of 950 356 California deliveries in 2007-2010 were analysed. Singleton live births of women whose prepregnancy body mass index (BMI) was underweight (<18.5 kg/m2) or normal (18.50-24.99 kg/m2) were analysed. Underweight BMI was further categorised as: severe (<16.00), moderate (16.00-16.99) or mild (17.00-18.49). PTB was grouped as 22-27, 28-31, 32-36 or <37 weeks (compared with 37-41 weeks). Adjusted multivariable Poisson regression modeling was used to estimate relative risk for PTB.
  
  
  
  Risk of PTB.
  
  
  
  About 72 686 (7.6%) women were underweight. Increasing severity of underweight was associated with increasing percent PTB: 7.8% (n = 4421) in mild, 9.0% (n = 1001) in moderate and 10.2% (475) in severe underweight. The adjusted relative risk of PTB also significantly increased: adjusted relative risk (aRR) = 1.22 (95% CI 1.19-1.26) in mild, aRR = 1.41 (95% CI 1.32-1.50) in moderate and aRR = 1.61 (95% CI 1.47-1.76) in severe underweight. These findings were similar in spontaneous PTB, medically indicated PTB, and the gestational age groupings.
  
  
  
  Increasing severity of maternal prepregnancy underweight BMI was associated with increasing risk-adjusted PTB at <37 weeks. This increasing risk was of similar magnitude in spontaneous and medically indicated births and in preterm delivery at 28-31 and at 32-36 weeks of gestation.
  
  
  
  Increasing severity of maternal underweight BMI was associated with increasing risk of preterm birth.
authors.affiliation: Department of Obstetrics & Gynecology, Stanford University School of Medicine, Stanford, CA, USA. anna.girsen@gmail.com.
authors.forename: A I
authors.initials: AI
authors.lastname: Girsen
authors.affiliation: Department of Pediatrics, Division of Neonatal and Developmental Medicine, Stanford University School of Medicine, Stanford, CA, USA.
authors.forename: J A
authors.initials: JA
authors.lastname: Mayo
authors.affiliation: Department of Pediatrics, Division of Neonatal and Developmental Medicine, Stanford University School of Medicine, Stanford, CA, USA.
authors.forename: S L
authors.initials: SL
authors.lastname: Carmichael
authors.affiliation: Department of Pediatrics, Division of Neonatal and Developmental Medicine, Stanford University School of Medicine, Stanford, CA, USA.|Health Economics Resource Center, Veterans Affairs Palo Alto Healthcare System, Menlo Park, CA, USA.
authors.forename: C S
authors.initials: CS
authors.lastname: Phibbs
authors.affiliation: Department of Pediatrics, Division of Neonatal and Developmental Medicine, Stanford University School of Medicine, Stanford, CA, USA.
authors.forename: B Z
authors.initials: BZ
authors.lastname: Shachar
authors.affiliation: Department of Pediatrics, Division of Neonatal and Developmental Medicine, Stanford University School of Medicine, Stanford, CA, USA.
authors.forename: D K
authors.initials: DK
authors.lastname: Stevenson
authors.affiliation: Department of Obstetrics & Gynecology, Stanford University School of Medicine, Stanford, CA, USA.
authors.forename: D J
authors.initials: DJ
authors.lastname: Lyell
authors.affiliation: Department of Pediatrics, Division of Neonatal and Developmental Medicine, Stanford University School of Medicine, Stanford, CA, USA.
authors.forename: G M
authors.initials: GM
authors.lastname: Shaw
authors.affiliation: Department of Pediatrics, Division of Neonatal and Developmental Medicine, Stanford University School of Medicine, Stanford, CA, USA.
authors.forename: J B
authors.initials: JB
authors.lastname: Gould
mesh_terms: D000328:Adult; D015992:Body Mass Index; D002140:California / Q000453:epidemiology; D005260:Female; D005865:Gestational Age; D006801:Humans; D007231:Infant, Newborn; D010298:Parity; D011247:Pregnancy; D047928:Premature Birth / Q000175:diagnosis* / Q000453:epidemiology / Q000209:etiology*; D012189:Retrospective Studies; D012307:Risk Factors; D012720:Severity of Illness Index; D013851:Thinness / Q000175:diagnosis* / Q000453:epidemiology
keywords: Pregnancy; preterm birth; underweight