A study of the epidemiology of breast cancer was conducted in São Paulo, Brazil, with data obtained from interviews of 536 cases of the disease and 1550 controls drawn from women attending the same hospitals as the breast cancer patients. Women who had had a first pregnancy prior to age 20 had only about two-thirds the breast cancer risk of those whose first pregnancy occurred after age 25. The association with age at first pregnancy appeared to explain at least part of the association of decreasing breast cancer risk with increasing parity. The protective effect of early pregnancy was confined to women under the age of 50 at the time of interview. Among women aged 50 or more, there was a statistically significant association of risk with increasing weight and score on the index weight/height2. No such associations were found among younger women. The cases and controls did not differ appreciably in any of several measures of lactation experience. The restriction to two different age groups of the two major positive findings of the study (association of risk with late first pregnancy and with weight) suggests that there may be two periods of life when a woman's risk of breast tumor induction is particularly high.
Work in São Paulo was supported by Grant R/00102 from the World Health Organization; that in Boston was supported by Grant E-385 A from the American Cancer Society, Inc., and Grant 5 PO1 CA 06373 from the National Cancer Institute, USPHS.