Increasing parity and duration of combined oral contraceptive (COC) use provide substantial protection against ovarian carcinoma (cancer). There are limited data on the impact of the age of the births or age of COC use on reducing ovarian cancer risk. Here, we examined the effects of age at first and last births and age at use of COCs using data from studies conducted in Los Angeles County, California, USA (1,632 cases, 2,340 controls). After adjusting for the number of births, every 5 years that a first birth was delayed reduced the risk of ovarian cancer by 13% (95% CI 5–21%; p = 0.003); a first birth after age 35 was associated with a 47% lower risk than a first birth before age 25. COC use before age 35 was associated with greater protection per year of use than COC use at older ages. Considering previously published results as well as the results presented here, increasing parity and a later age at births are both important protective factors against ovarian cancer and the protection extends over 30 or more years from last birth. Current models of the etiology of ovarian cancer do not encompass an effect of late age at births. Our result of an attenuation of the protective effect with COC use after around age 35 needs further investigation as it has not been seen in all studies.
