Abstract
We conducted a population-based case-control study of breast cancer among Chinese-, Japanese- and Filipino-American women in Los Angeles County Metropolitan Statistical Area (MSA), San Francisco-Oakland MSA and Oahu, Hawaii. One objective of the study was to quantify breast cancer risks in relation to menstrual and reproductive histories in migrant and US-born Asian-Americans and to establish whether the gradient of risk in Asian-Americans can be explained by these factors. Using a common study design and questionnaire in the three study areas, we successfully conducted in-person interviews with 597 Asian-American women diagnosed with incident, primary breast cancer during the period 1983-87 (70% of those eligible) and 966 population-based controls (75% of those eligible). Controls were matched to cases on age, ethnicity and area of residence. In the present analysis, which included 492 cases and 768 controls, we observed a statistically non-significant 4% reduction in risk of breast cancer with each year delay in onset of menstruation. Independent of age at menarche risk of breast cancer was lower (odds ratio; OR=0.77) among women with menstrual cycles greater than 29 days. Parous Asian-American women showed a significantly lower risk of breast cancer then nulliparous women (OR=0.54). An increasing number of livebirths and a decreasing age at first livebirth were both associated with a lower risk of breast cancer, although the effect of number of livebirths was no longer significant after adjustment for age at first livebirth. Women with a pregnancy (spontaneous or induced abortions) but no livebirth had a statistically non-significant increased risk (OR=1.84), but there was no evidence that one type of abortion was particularly harmful. A positive history of breastfeeding was associated with non-significantly lower risk of breast cancer (OR=.78). There are several notable differences in the menstrual and reproductive factors between Asian-Americans in this study and published data on US whites. US-born Asian Americans had an average age at menarche of 12.12 years-no older than has been found in comparable studies of US whites, but 1.4 years earlier than Asian women who migrated to the US. Asian-American women, particularly those born in the US and those who migrated before age 36, also had a later age at first birth and fewer livebirths than US whites. A slightly higher proportion of Asian-American women breastfed, compared with US whites. The duration of breastfeeding was similar in US-born Asians and US whites, but was longer in Asian migrants, especially those who migrated at a later age. Menstrual and reproductive factors in Asian-American women are consistent with their breast cancer rates being at least as high as in US whites, and they are. However, the effects of these menstrual and reproductive factors were small and the ORs for migration variables changed only slightly after adjustment for these menstrual and reproductive factors. These results suggest that the lower rates of breast cancer in Asians must be largely as a result of other environmental/lifestyle factors.
Highlights
The importance of menstrual and reproductive factors as determinants of a woman's risk of breast cancer is well established (MacMahon et al, 1970; Henderson et al, 1984; Kelsey et al, 1993)
We examine the role of menstrual and reproductive factors in a population-based case-control study of breast cancer in Chinese, Japanese- and Filipino-Americans living in the San Francisco-Oakland Metropolitan Statistical Area (MSA), Los Angeles MSA or Oahu, Hawaii
Compared with Asian-American women born in the West, Asian women who migrated to the US had later age at menarche, reported slightly more pregnancies and more livebirths and breastfed for a considerably longer time
Summary
The importance of menstrual and reproductive factors (age at menarche, age at first birth, parity, age at and type of menopause) as determinants of a woman's risk of breast cancer is well established (MacMahon et al, 1970; Henderson et al, 1984; Kelsey et al, 1993). These factors have been found to be important in high-risk Western populations (Kvale, 1992) and in low-risk Asian groups (Tao et al, 1988; Yuan et al, 1988; Wang et al, 1992), the magnitude of risks and the relative importance of specific factors have compare varied in different studies. The extent to which changes in menstrual and reproductive factors can explain the increase in incidence rates of breast cancer in Asian-
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