title: Maternal underweight and the risk of preterm birth and low birth weight: a systematic review and meta-analyses.
abstract: BACKGROUND
  Despite the current obesity epidemic, maternal underweight remains a common occurrence with potential adverse perinatal outcomes. Our objective was to determine the relationship between maternal underweight and preterm birth (PTB) and low birth weight (LBW) in singleton pregnancies in developing and developed countries.
  
  
  METHODS
  We followed the MOOSE consensus statement. We searched MEDLINE and EMBASE from their inceptions. We included studies that assessed the effect of maternal underweight compared with normal weight according to body mass index in singleton gestations on our two primary outcomes: PTB (<37 weeks) and LBW (<2500 g). Two assessors independently reviewed citations, extracted data and assessed quality.
  
  
  RESULTS
  A total of 78 studies were included involving 1 025 794 women. The overall risk of PTB was increased in the cohort studies of underweight women [adjusted relative risk (RR) 1.29, 95% confidence interval (CI) 1.15-1.46], as were the risks of spontaneous PTB (adjusted RR 1.32, 95% CI 1.10-1.57) and induced PTB (adjusted RR 1.21, 95% CI 1.07-1.36). Underweight women had an increased risk of an LBW infant (adjusted RR 1.64, 95% CI 1.38-1.94). In developed countries, underweight women had an increased risk of PTB (RR 1.22, 95% CI 1.15-1.30) but not in developing countries (RR 0.99, 95% CI 0.67-1.45). In both developed and developing countries, underweight women were at increased risk of having an LBW infant (RR 1.48, 95% CI 1.29-1.68, and RR 1.52, 95% CI 1.25-1.85, respectively).
  
  
  CONCLUSIONS
  In this systematic review and meta-analyses, we determined that singletons born to underweight women have higher risks of PTB (overall, spontaneous and induced) and LBW than those born to women with normal weight.
authors.affiliation: Department of Obstetrics and Gynecology, First Affiliated Hospital of Xi'an Jiao Tong University, Shaanxi Province, PR China.
authors.forename: Zhen
authors.initials: Z
authors.lastname: Han
authors.forename: Sohail
authors.initials: S
authors.lastname: Mulla
authors.forename: Joseph
authors.initials: J
authors.lastname: Beyene
authors.forename: Grace
authors.initials: G
authors.lastname: Liao
authors.forename: Sarah D
authors.initials: SD
authors.lastname: McDonald
mesh_terms: D015992:Body Mass Index; D005260:Female; D006801:Humans; D007230:Infant, Low Birth Weight; D007231:Infant, Newborn; D009035:Mothers; D011247:Pregnancy; D011256:Pregnancy Outcome; D047928:Premature Birth / Q000453:epidemiology*; D012307:Risk Factors; D013851:Thinness