OBJECTIVES: There has been speculation that women with oestrogen-related cancers have been exposed to high levels of oestrogen in earlier life and therefore also during their pregnancies prior to the disease. For this reason we examined signs of reproductive outcome related to oestrogen exposure such as twinning, sex ratio, birth weight and congenital malformation in women who later were diagnosed with breast cancer, endometrial or ovarian cancer. METHODS: For 5213 children born to women who later developed breast cancer, 557 children born to women who later developed ovarian cancer and 173 children born to mothers with endometrial cancers we selected as a control four times as many newborns of mothers without these cancers after matching for mothers' parity, age, date of birth and hospital attended. All data were extracted from existing nationwide files. Most of the women's reproductive lives fell within the period of the birth registry from 1973 to 1993. RESULTS: No differences in sex ratio, birth weight or malformation were found between the newborns to cancer or control mothers in any of the cancer groups studied. The only significant findings were less than expected births of twins to mothers who later developed cancer of the ovaries and a higher proportion of newborns with a birth weight over 4000 g in the cancer corpus uterus group. CONCLUSION: Although the amount of information is limited for endometrial and ovarian cancers, we found no strong indication of elevated oestrogen levels during pregnancy.