title: Multiparity and the risk of premenopausal breast cancer: different effects across ethnic groups in Singapore.
abstract: BACKGROUND
  The relationship between multiparity and premenopausal breast cancer risk is different in Caucasian, African-American and Hispanic women. For Asian women, this relationship has never been well studied.
  
  
  METHODS
  Within the Singapore Birth Registry, we selected all women who had a first child between 1986 and 2002 (169,936 Chinese, 40,521 Malay, 17,966 Indian). We linked them to the Singapore Cancer Registry data to identify those who developed breast cancer after childbirth (n = 527). We used multivariate Cox analysis to examine the relationship between parity, ethnicity and premenopausal breast cancer risk.
  
  
  RESULTS
  Compared to Chinese, Malay women had increased and Indian women had decreased risks of premenopausal breast cancer (adjusted Hazard Ratios [HRadj] 1.25 [1.0-1.6] and 0.48 [0.3-0.8] respectively). Multiparity did not modify the risk of premenopausal breast cancer in Chinese and Indians. In Malays there was a significant risk reduction with increasing parity (P (trend )0.037). Malay women with one, two and >or=3 children had premenopausal breast cancer risks (HR(adj)) of 1.86 (1.2-3.0), 1.52 (1.1-2.2) and 0.87 (0.6-1.3) respectively compared to their Chinese counterparts.
  
  
  CONCLUSIONS
  The impact of multiparity on premenopausal breast cancer risk differs across ethnic groups in Singapore. Increasing parity reduces the risk of premenopausal breast cancer in Malay, but not in Chinese and Indian women. Uniparous Malay women have twice the risk of premenopausal breast cancer compared to uniparous Chinese. This excess risk disappears after giving birth to >or=3 children. Indian women have lower premenopausal breast cancer risks than Chinese, regardless of their parity status.
authors.affiliation: Centre for Molecular Epidemiology, Faculty of Medicine, National University of Singapore, Singapore. cofhmv@nus.edu.sg
authors.forename: Helena M
authors.initials: HM
authors.lastname: Verkooijen
authors.forename: Karen P L
authors.initials: KP
authors.lastname: Yap
authors.forename: Vineta
authors.initials: V
authors.lastname: Bhalla
authors.forename: Khuan Yew
authors.initials: KY
authors.lastname: Chow
authors.forename: Kee Seng
authors.initials: KS
authors.lastname: Chia
mesh_terms: D000328:Adult; D001943:Breast Neoplasms / Q000453:epidemiology* / Q000208:ethnology*; D005006:Ethnicity; D005260:Female; D006801:Humans; D008875:Middle Aged; D010298:Parity; D011247:Pregnancy; D017697:Premenopause; D012042:Registries; D012307:Risk Factors; D012846:Singapore / Q000453:epidemiology / Q000208:ethnology; D055815:Young Adult