Hormone-related cancers may originate in utero. Accumulating evidence is given by animal studies, descriptive epidemiologic data and analytic etiologic studies. Indicators of high levels of endogenous pregnancy hormones, like high birth weight and jaundice in the offspring, are associated with increased risk for breast, prostate, and non-seminoma testicular cancer. Indicators of low levels, like pre-eclampsia, are associated with decreased risk. These results support the hypothesis that the pre- and perinatal periods constitute a 'time window' that is of major importance for future cancer risk.