ObjectiveTo assess the influence of postnatal factors on the development of clear cell adenocarcinoma in women exposed to diethylstilbesterol (DES), particularly factors related to exogenous or endogenous hormone exposures, and to reassess the relation of season of birth.MethodsFor the analysis of postnatal factors, 244 cases were compared with 244 age-matched DES-exposed women. Information was obtained from telephone interviews, with questions asked in reference to an index age, the age at which clear cell adenocarcinoma was diagnosed. For the analysis of season of birth, 604 cases, living or deceased, were compared with 1749 DES-exposed women.ResultsNeither oral contraceptive (OC) use nor pregnancy was associated with risk of clear cell adenocarcinoma: the odds ratios (OR) were 1.1 (95% confidence interval [CI] 0.7, 1.8) for OC use and 1.3 (95% CI 0.7, 2.3) for pregnancy. The OR for fall season of birth relative to all other seasons was 1.2 (95% CI 0.9, 1.4).ConclusionPregnancy and OCs do not increase risk of DES-associated clear cell adenocarcinoma. In addition, season of birth does not appear to be a material factor; the previous finding of an increased risk for fall season of birth is most likely not valid and probably a chance finding due to small sample size.