Increasing epidemiological and experimental evidence indicates that the carcinogenic pathway in the breast and female reproductive organs is driven, at least in part, by factors associated with reproduction. We conducted a retrospective cohort study, comparing the risk of ovarian, breast, endometrial, and cervical cancers among women who had records of at least one twin pregnancy, compared with women who had given birth to only single children. Subjects were selected from the Utah Population Database, which consists of multiple linked datasets including genealogy, births and deaths and cancer registries. We used Poisson regression to calculate relative risks, adjusted for the number of pregnancies and the age of the mother at the birth of first and last children, with singleton mothers as the reference group in each case. The risks of breast and ovarian cancers did not differ between mothers of twins and mothers of single children. The risk of endometrial cancer was slightly lower in mothers of twins than in mothers of singleton children (RR = 0.90, 95% CI 0.67-1.21). Conversely the risk of cervical cancer was higher among twin mothers (RR = 1.78, 95% CI 0.88-3.52). This latter finding supports previous data suggesting that reproductive hormones act as cofactors in the etiology of cervical cancer.