Age at first birth and the risk of epithelial ovarian cancer

The relationships between age at first birth, parity, and the risk of ovarian cancer were evaluated in a case-control study of 272 women with histologically confirmed epithelial ovarian cancer and 544 age-matched controls with a spectrum of acute conditions unrelated to any of the established or potential risk factors for ovarian cancer. Late age at first birth was associated with increased risk: Compared to women who first had a child before the age of 22 years, the relative risks (RR) for those who first gave birth at ages 22-24, 25-27, and 28 or more were 2.7, 3.2, and 4.0, respectively. Nulliparous women showed increased RR (3.9) comparable to the RR among women who first bore a child at age 25 or more, regardless of the number of births. The elevated risk associated with later age at first birth was not accounted for by low parity. The risk of ovarian cancer, as expected, increased with decreasing parity: RR estimates for women having 5 or more, 3 or 4, and 1 or 2 children and for nulliparae were 1.0, 1.7, 1.9, and 2.6, respectively. However, the inverse association between parity and ovarian cancer could be accounted for largely by the importance of age at first birth, because when adjustment was made for that variable, the RR for 3 or 4 and 1 or 2 children decreased to 1.3 and 1.2, respectively. Thus the results of the present study show a strong independent effect of age at first birth on the risk of epithelial ovarian cancer, whereas the association with parity can be explained largely or totally in terms of a high correlation between total parity and age at first birth. The pattern of ovarian cancer risk that emerges from this study, therefore, is similar to the epidemiology of breast cancer. General evidence on this issue from various other studies, however, is rather controversial, and similar analysis of other data-sets would be useful.