Breast cancer and oral contraceptive use in Asian-American women

Breast cancer incidence has historically been 4-7 times higher in the Unite d States than in Asia. A previous study by the authors in Asian-American wo men demonstrated a substantial increase in breast cancer risk in women who migrated from Asia to the United States, with the risk almost doubling duri ng the first decade after migration. Increased use of oral contraceptives s oon after migration to the United States could possibly explain this rapid rise in risk. In a population-based case-control study of Chinese, Filipino , and Japanese-American women, aged 20-55 years, who lived in San Francisco -Oakland, California; Los Angeles, California; and Oahu, Hawaii during 1983 -1987, 597 cases (70% of those eligible) and 966 controls (75%) were interv iewed. Controls were matched to cases on age, ethnicity, and area of reside nce. Oral contraceptive (OC) use increased with time since migration; 15.0% of Asian-born women who had been in the West <8 years, 33.4% of Asian-born women who had been in the West greater than or equal to 8 years, and 49.6% of Asian women born in the West had ever used OCs. However, duration of OC use (adjusted for age, ethnicity, study area, years since migration, educa tion, family history of breast cancer and age at first full-term birth) was not associated with increased risk of breast cancer. Moreover, neither OC use before age 25 years nor before first full-term birth was associated wit h increased risk. Results were unchanged when restricted to women under age 45 years or under age 40 years. After adjustment for duration of OC use, w omen who had been in the United States greater than or equal to 8 years wer e still at almost twice the risk of breast cancer compared with women who h ad been in the United States 2-7 years. This study suggests that OC use can not explain the elevated risk observed in Asian women who migrated to the U nited States greater than or equal to 7 years ago.