MULTIPLE BIRTHS, SEX OF CHILDREN AND SUBSEQUENT BREAST-CANCER RISK FOR THE MOTHERS - A PROSPECTIVE-STUDY IN NORWAY

Endocrinological changes occurring during pregnancy may influence the subsequent cancer risk of the mother. Further, the endocrinological mi lieu may differ according to different birth characteristics. In the p resent study possible relations between multiple births, sex of childr en and breast-cancer risk were examined in a population-based, prospec tive study of 802,269 parous Norwegian women aged 20-56 years. A total of 4,782 women were diagnosed with breast cancer during follow-up. Of these, 97 had ever experienced a multiple birth. We found a slightly lower risk of breast cancer among women ever having had a multiple bir th than among women with singletons only (IRR = 0.89, 95% CI = 0.73-1. 09). The reduction in risk was mainly observed among women with a mult iple last birth. Further, the reduction in risk seemed to diminish wit h increasing parity, and among women with 4 or more full-term pregnanc ies, ever having had a multiple birth was associated with an elevated risk (IRR = 1.48, 95% CI = 0.97-2.25). The sex of the first or last ch ild did not affect the subsequent breast-cancer risk. Further, we foun d no associations with the sex distribution among all children or in m ultiple births, despite a tendency toward a reduced risk among women w ith several daughters only. (C) 1995 Wiley-Liss, Inc.