Birth characteristics and breast cancer risk: a study among like-sexed twins.

High birth weight has been proposed to increase the risk of breast cancer. Birth weight and gestational age are positively associated with intrauterine estrogen exposure, which may initiate the carcinogenic process in mammary gland tissue and determine mammary gland cell mass later in life. In the present case-control study, we retrieved birth records for 96 female twin pairs where one twin had developed breast cancer. We used two control groups. In the external comparison, we used aged-matched twins as the control group, and in the within-pair comparison the control group included the healthy twin sisters. Odds ratio (OR) was used to estimate the relative risk, using 95% confidence intervals (CI). Compared with twins with gestational age less than 33 weeks, twins with gestational age of more than 40 weeks were at increased risk of breast cancer (OR = 8.4; 95% CI 1.3-54.4). In the within-pair comparison, mean birth weight and ponderal index were higher among cases compared with their co-twins (p = 0.07 and p = 0.05, respectively). The risk of breast cancer increased with birth weight. Compared with women with birth weight of less than 2,000 g, women with birth weight of at least 3,000 g had a nonsignificant increase in risk of developing breast cancer (OR = 3.5; 95% CI 0.7-18.5). The results give some support to the hypothesis that the fetal environment influences adult breast cancer risk. Copyright 2001 Wiley-Liss, Inc.