title: Birth intervals and breast cancer risk.
abstract: BACKGROUND
  The interval between successive births (birth interval) may affect breast cancer risk, whereas interval from last birth to cancer onset may modify its behaviour.
  
  
  METHODS
  The study cohort consisted of 29 488 Finnish grand multiparous (GM) women, including 628 women with breast cancer. Conditional logistic regression for case-control design nested within the cohort was used to estimate proportional hazards (referred as relative risks, RR). Age at first birth and parity were co-variables.
  
  
  RESULTS
  Short interval (<1 year) between first and second birth increased the risk of advanced ductal breast cancer at ages < 50 years (RR=5.29; 95% CI 2.00-14.0) as compared to interval 3+ years. The risk of advanced ductal cancer was also large (RR = 4.00; 95% CI 1.19-13.4) shortly (<3 years) after last birth as compared with the period 15+ years.
  
  
  CONCLUSIONS
  Short birth interval-associated excess breast cancer risk may be related to stimulatory effects of female steroid hormones produced during two closely connected pregnancies, or defective breast maturation owing to failures in breastfeeding.
authors.affiliation: Department of Obstetrics and Gynecology, Oulu University Hospital, FI-90250 Oulu, Finland.
authors.forename: A
authors.initials: A
authors.lastname: Kauppila
authors.forename: P
authors.initials: P
authors.lastname: Kyyrönen
authors.forename: M
authors.initials: M
authors.lastname: Hinkula
authors.forename: E
authors.initials: E
authors.lastname: Pukkala
mesh_terms: D000328:Adult; D000368:Aged; D001721:Birth Intervals; D001943:Breast Neoplasms / Q000209:etiology*; D018270:Carcinoma, Ductal, Breast / Q000209:etiology; D016022:Case-Control Studies; D005260:Female; D006801:Humans; D007774:Lactation; D016015:Logistic Models; D008423:Maternal Age; D008875:Middle Aged; D011247:Pregnancy; D011388:Prolactin / Q000502:physiology
chemical_list: D011388:Prolactin