MENSTRUAL AND REPRODUCTIVE FACTORS AND RISK OF BREAST-CANCER IN ASIAN-AMERICANS

We conducted a population-based case-control study of breast cancer am ong Chinese-, Japanese- and Filipino-American women in Los Angeles Cou nty Metropolitan Statistical Area (MSA), San Francisco-Oakland MSA and Oahu, Hawaii. One objective of the study was to quantify breast cance r 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 com mon study design and questionnaire in the three study areas, we succes sfully conducted in-person interviews with 597 Asian-American women di agnosed with incident, primary breast cancer during the period 1983-87 (70% of those eligible) and 966 population-based controls (75% of tho se eligible). Controls were matched to cases on age, ethnicity and are a 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 menstruatio n. Independent of age at menarche risk of breast cancer was lower (odd s ratio; OR= 0.77) among women with menstrual cycles greater than 29 d ays. Parous Asian-American women showed a significantly lower risk of breast cancer than nulliparous women (OR = 0.54). An increasing number of livebirths and a decreasing age at first livebirth were both assoc iated with a lower risk of breast cancer, although the effect of numbe r of livebirths was no longer significant after adjustment for age at first livebirth. Women with a pregnancy (spontaneous or induced aborti ons) but no livebirth had a statistically non-significant increased ri sk (OR = 1.84), but there was no evidence that one type of abortion wa s particularly harmful. A positive history of breastfeeding was associ ated with non-significantly lower risk of breast cancer (OR = 0.78). T here are several notable differences in the menstrual and reproductive factors between Asian-Americans in this study and published data on U S whites. US-born Asian-Americans had an average age at menarche of 12 .2 years-no older than has been found in comparable studies of US whit es, but 1.4 years earlier than Asian women who migrated to the US. Asi an-American women, particularly those born in the US and those who mig rated before age 36, also had a later age at first birth and fewer liv ebirths than US whites. A slightly higher proportion of Asian-American women breastfed, compared with US whites. The duration of breastfeedi ng was similar in US-born Asians and US whites, but was longer in Asia n migrants, especially those who migrated al a later age. Menstrual an d reproductive Factors in Asian-American women are consistent with the ir boast 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 r esults suggest that the lower rates of breast cancer in Asians must be largely as a result of other environmental/lifestyle factors.