title: Interpregnancy weight gain and the male-to-female sex ratio of the second pregnancy: a population-based cohort study.
abstract: OBJECTIVE
  To investigate whether interpregnancy maternal weight change (difference between body mass index [BMI] at the first antenatal visit of the second pregnancy and BMI at the first antenatal visit of the first pregnancy) or changes in smoking status between pregnancies is related to the sex ratio of the second pregnancy.
  
  
  METHODS
  Population-based cohort study.
  
  
  METHODS
  Swedish Birth Registry.
  
  
  METHODS
  A total of 220,889 women who had their first two consecutive singleton births between 1992 and 2004. Both live births and stillbirths were included.
  
  
  METHODS
  Analyses of data collected prospectively in nationwide registries.
  
  
  METHODS
  Male-to-female sex ratio of the second pregnancy.
  
  
  RESULTS
  The sex ratio of the second pregnancy increased linearly with the amount of maternal weight change from the first to the second pregnancies, from 1.024 in women who lost more than 1 unit BMI to 1.080 in women who gained 3 or more units. This association was independent of obstetric complications, length of the interpregnancy interval, and maternal sociodemographic characteristics at the second pregnancy. Change in maternal smoking status from the first to the second pregnancies was not significantly related to the sex ratio of the second pregnancy.
  
  
  CONCLUSIONS
  There could be a causal relation between prepregnant maternal weight gain and the sex ratio of the offspring.
authors: Departments of Nutrition and Epidemiology, Harvard School of Public Health, Boston, Massachusetts. Electronic address: evillamo@hsph.harvard.edu.
authors: Eduardo
authors: E
authors: Villamor
authors: Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Stockholm, Sweden.
authors: Pär
authors: P
authors: Sparén
authors: Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Stockholm, Sweden.
authors: Sven
authors: S
authors: Cnattingius
mesh_terms: D000328:Adult; D001721:Birth Intervals; D015992:Body Mass Index; D015331:Cohort Studies; D005260:Female; D006801:Humans; D016015:Logistic Models; D008297:Male; D011247:Pregnancy; D011446:Prospective Studies; D012042:Registries; D012744:Sex Ratio; D012907:Smoking / Q000503:physiopathology; D013548:Sweden; D013997:Time Factors; D015430:Weight Gain / Q000502:physiology*