title: Dual effect of short interval between first and second birth on ductal breast cancer risk in Finland.
abstract: OBJECTIVE
  A short interval between the first and second birth was associated with an increased risk of advanced ductal breast cancer among women with 5+ childbirths in our previous study. We now evaluated the significance of this risk factor and its relation to the age at first birth among mothers with 2-4 children.
  
  
  METHODS
  The cohort of 190,949 Finnish women with 2-4 children comprised 3,834 women with ductal breast cancer diagnosed before 2009. Conditional logistic regression for case-control design nested within the cohort was used to estimate proportional hazard ratios (HR) associated with the birth interval. Controls were matched for age and number of children. Age at the first birth and the interval from the last birth to cancer were co-variables.
  
  
  RESULTS
  Among women with the first birth <30 years, the HR of advanced ductal breast cancer at 50+ years for a short (<1.5 years) versus long (>3 years) interval between the first and second birth was 0.48 (95% Confidence Interval 0.33-0.70). Among women with the first birth at 30+ years, the HR of this cancer type diagnosed before the age of 50 years for a short versus long interval between the first and second birth was 5.83 (95% CI 2.30-14.8).
  
  
  CONCLUSIONS
  The interval between first and second birth strongly influences the risk of ductal breast cancer. Because second pregnancy soon after the first one decreased the risk of ductal breast cancer in young primiparas but increased the risk in older primiparas, it is likely that in such circumstances second pregnancy continues the actions initiated by the first pregnancy/breast-feeding.
authors.affiliation: Department of Obstetrics and Gynaecology, Oulu University Hospital, 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: Lehtinen
authors.forename: E
authors.initials: E
authors.lastname: Pukkala
mesh_terms: D000328:Adult; D000367:Age Factors; D000368:Aged; D000369:Aged, 80 and over; D001721:Birth Intervals / Q000706:statistics & numerical data*; D001722:Birth Order; D001943:Breast Neoplasms / Q000453:epidemiology* / Q000209:etiology; D018270:Carcinoma, Ductal, Breast / Q000453:epidemiology* / Q000209:etiology; D016022:Case-Control Studies; D015331:Cohort Studies; D005260:Female; D005387:Finland / Q000453:epidemiology; D006801:Humans; D008875:Middle Aged; D011247:Pregnancy; D011252:Pregnancy Complications, Neoplastic / Q000453:epidemiology; D012307:Risk Factors; D055815:Young Adult