title: Placental weight and risk of invasive epithelial ovarian cancer with an early age of onset.
abstract: BACKGROUND
  Epithelial ovarian cancer is associated with reproductive factors, but we lack knowledge if hormonal factors during pregnancy influence the mother's risk. Because pregnancy hormones are primarily produced by the placenta, placental weight may be an indirect marker of hormone exposure during pregnancy.
  
  
  METHODS
  In a nationwide Swedish cohort study, we included women with singleton births from 1982 to 1989. Women were followed for occurrence of invasive epithelial ovarian cancer, death, or emigration through 2004. Hazard ratios (HR) with 95% confidence intervals (95% CI) from Cox models were used to estimate associations between pregnancy exposures and epithelial ovarian cancer.
  
  
  RESULTS
  Among 395,171 women with information on placental weight in their first recorded birth, 316 women developed invasive epithelial ovarian cancer. Mean age at diagnosis was 44 years. Compared with women with a placental weight of 500 to 699 g, women with a high (>or=700 g) placental weight had an increased risk of developing epithelial ovarian cancer (HR, 1.47; 95% CI, 1.14-1.90). Compared with women with term pregnancies (40-41 weeks), women with post-term (>or=42 weeks) pregnancies had an increased risk of developing epithelial ovarian cancer (HR, 1.48; 95% CI, 1.00-2.19). These associations were slightly stronger when we included information about women's overall first birth, and slightly weaker when we included information about last recorded birth or ever last birth from 1982 to 1989.
  
  
  CONCLUSIONS
  Because pregnancy hormone levels increase with placental weight, our study supports the hypothesis that hormone exposures during pregnancy influence the risk of invasive epithelial ovarian cancer among young women.
authors.affiliation: Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Stockholm, Sweden. sven.cnattingius@ki.se
authors.forename: Sven
authors.initials: S
authors.lastname: Cnattingius
authors.forename: Sandra
authors.initials: S
authors.lastname: Eloranta
authors.forename: Hans-Olov
authors.initials: HO
authors.lastname: Adami
authors.forename: Ove
authors.initials: O
authors.lastname: Axelsson
authors.forename: Paul W
authors.initials: PW
authors.lastname: Dickman
authors.forename: Chung-cheng
authors.initials: CC
authors.lastname: Hsieh
authors.forename: Lorelei A
authors.initials: LA
authors.lastname: Mucci
authors.forename: Dimitrios
authors.initials: D
authors.lastname: Trichopoulos
authors.forename: Mats
authors.initials: M
authors.lastname: Lambe
authors.forename: Anna L V
authors.initials: AL
authors.lastname: Johansson