title: Pregnancy characteristics and maternal risk of breast cancer.
abstract: BACKGROUND
  During pregnancy, serum levels of estrogen, progesterone, and other hormones are markedly higher than during other periods of life. Pregnancy hormones primarily are produced in the placenta, and signs of placental impairment may serve as indirect markers of hormone exposures during pregnancy. During pregnancy, these markers have been inconsistently associated with subsequent risk of breast cancer in the mother.
  
  
  OBJECTIVE
  To examine associations between indirect markers of hormonal exposures, such as placental weight and other pregnancy characteristics, and maternal risk of developing breast cancer.
  
  
  METHODS
  Population-based cohort study using data from the Swedish Birth Register, the Swedish Cancer Register, the Swedish Cause of Death Register, and the Swedish Register of Population and Population Changes.
  
  
  METHODS
  Women included in the Sweden Birth Register who delivered singletons between 1982 and 1989, with complete information on date of birth and gestational age. Women were followed up until the occurrence of breast cancer, death, or end of follow-up (December 31, 2001). Cox proportional hazards models were used to estimate associations between hormone exposures and risks of breast cancer.
  
  
  METHODS
  Incidence of invasive breast cancer.
  
  
  RESULTS
  Of 314,019 women in the cohort, 2216 (0.7%) developed breast cancer during the follow-up through 2001, of whom 2100 (95%) were diagnosed before age 50 years. Compared with women who had placentas weighing less than 500 g in 2 consecutive pregnancies, the risk of breast cancer was increased among women whose placentas weighed between 500 and 699 g in their first pregnancy and at least 700 g in their second pregnancy (or vice versa) (adjusted hazard ratio, 1.82; 95% confidence interval [CI], 1.07-3.08), and the corresponding risk was doubled among women whose placentas weighed at least 700 g in both pregnancies (adjusted hazard ratio, 2.05; 95% CI, 1.15-3.64). A high birth weight (> or =4000 g) in 2 successive births was associated with an increased risk of breast cancer before but not after adjusting for placental weight and other covariates (adjusted hazard ratio, 1.10; 95% CI, 0.76-1.59).
  
  
  CONCLUSIONS
  Placental weight is positively associated with maternal risk of breast cancer. These results further support the hypothesis that pregnancy hormones are important modifiers of subsequent maternal breast cancer risk.
authors.affiliation: Department of Medical Epidemiology and Biostatistics, Karolinska Hospital, Karolinska Institutet, Stockholm, Sweden. sven.cnattingius@mep.ki.se
authors.forename: Sven
authors.initials: S
authors.lastname: Cnattingius
authors.forename: Anna
authors.initials: A
authors.lastname: Torrång
authors.forename: Anders
authors.initials: A
authors.lastname: Ekbom
authors.forename: Fredrik
authors.initials: F
authors.lastname: Granath
authors.forename: Gunnar
authors.initials: G
authors.lastname: Petersson
authors.forename: Mats
authors.initials: M
authors.lastname: Lambe
mesh_terms: D001724:Birth Weight; D001943:Breast Neoplasms / Q000453:epidemiology*; D015331:Cohort Studies; D004967:Estrogens / Q000378:metabolism*; D005260:Female; D006801:Humans; D007231:Infant, Newborn; D009929:Organ Size; D010298:Parity; D010920:Placenta; D010926:Placental Hormones / Q000378:metabolism*; D010929:Placentation; D011247:Pregnancy / Q000502:physiology*; D011374:Progesterone / Q000378:metabolism*; D016016:Proportional Hazards Models; D012042:Registries; D012306:Risk; D013548:Sweden
chemical_list: D004967:Estrogens; D010926:Placental Hormones; D011374:Progesterone