title: Smoking duration before first childbirth: an emerging risk factor for breast cancer? Results from 302,865 Norwegian women.
abstract: OBJECTIVE
  Recently, The International Agency for Research on Cancer classified cigarette smoking as possibly carcinogenic to the human breast. Since some new cohort studies have suggested that this risk is confined to women who started to smoke before first childbirth, we wanted to examine the association between smoking and breast cancer, with a focus on time of smoking initiation in relation to the first childbirth.
  
  
  METHODS
  We followed 302,865 Norwegian women born between 1899 and 1975, recruited from 1974 to 2003, by linkage to national registries through December 2007. We used Cox proportional hazard models to estimate hazard ratios (HR) and 95% confidence intervals (CI).
  
  
  RESULTS
  During more than 4.1 million person-years of follow-up, we ascertained 7,490 cases of primary invasive breast cancer. Compared with never smokers, ever smokers had a 15% (HR = 1.15, 95% CI 1.10-1.21) increased risk of breast cancer overall and also a significantly increased risk of breast cancer in the three most exposed categories of age at smoking initiation (parous women), number of cigarettes smoked per day, years of smoking duration and number of pack-years. Ever smokers who started to smoke more than 1 year after the first childbirth had not an increased risk (HR = 0.93, 95% CI 0.86-1.02), while those who initiated smoking more than 10 years before their first childbirth had a 60% (HR = 1.60, 95% CI 1.42-1.80) increased risk of breast cancer, compared with never smokers.
  
  
  CONCLUSIONS
  Smoking initiation before the first childbirth increases the risk of breast cancer.
authors: Department of Community Medicine, Faculty of Health Sciences, University of Tromsø, Tromsø, Norway.
authors: Eivind
authors: E
authors: Bjerkaas
authors: Ranjan
authors: R
authors: Parajuli
authors: Elisabete
authors: E
authors: Weiderpass
authors: Anders
authors: A
authors: Engeland
authors: Gertraud
authors: G
authors: Maskarinec
authors: Randi
authors: R
authors: Selmer
authors: Inger Torhild
authors: IT
authors: Gram
mesh_terms: D000328:Adult; D000368:Aged; D001943:Breast Neoplasms / Q000453:epidemiology* / Q000209:etiology; D015331:Cohort Studies; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D008875:Middle Aged; D009664:Norway; D036801:Parturition; D018570:Risk Assessment; D012907:Smoking / Q000009:adverse effects*
references: Breast Cancer Res Treat. 2002 Sep;75(2):181-4
references: 12243511
references: BMJ. 2011 Mar 01;342:d1016
references: 21363864
references: Cancer Epidemiol Biomarkers Prev. 2013 May;22(5):862-71
references: 23632818
references: Tidsskr Nor Laegeforen. 1991 Jun 30;111(17):2063-72
references: 1871734
references: Cancer Epidemiol Biomarkers Prev. 2004 Mar;13(3):398-404
references: 15006915
references: Int J Cancer. 2010 Dec 15;127(12):2893-917
references: 21351269
references: Epidemiology. 2002 Nov;13(6):745-6
references: 12410024
references: Am J Epidemiol. 2007 Jul 1;166(1):55-61
references: 17426039
references: Int J Epidemiol. 2010 Jun;39(3):660-7
references: 19380371
references: Breast Cancer Res. 2005;7(3):131-42
references: 15987443
references: Int J Epidemiol. 2008 Jun;37(3):481-5
references: 17984119
references: Environ Mol Mutagen. 2002;39(2-3):119-26
references: 11921179
references: Int J Epidemiol. 2011 Dec;40(6):1667-77
references: 21984693
references: Br J Cancer. 1988 Dec;58(6):832-7
references: 3224085
references: Am J Epidemiol. 2002 Oct 1;156(7):616-26
references: 12244030
references: Mayo Clin Proc. 2005 Nov;80(11):1423-8
references: 16295021
references: Breast Cancer Res Treat. 1982;2(1):5-73
references: 6216933
references: Breast Cancer Res Treat. 2006 Dec;100(3):293-9
references: 16773435
references: Arch Intern Med. 2011 Jan 24;171(2):125-33
references: 21263102
references: Eur J Cancer. 2009 May;45(7):1218-1231
references: 19091545
references: Am J Epidemiol. 2011 Aug 15;174(4):390-402
references: 21719745
references: Br J Cancer. 2004 Aug 2;91(3):512-8
references: 15226777
references: J Natl Cancer Inst. 2004 Jan 7;96(1):29-37
references: 14709736
references: Cancer Epidemiol Biomarkers Prev. 2005 Jan;14(1):61-6
references: 15668477
references: Br J Cancer. 2002 Nov 18;87(11):1234-45
references: 12439712
references: Acta Med Scand Suppl. 1979;634:1-70
references: 293122