Age at first birth, parity, and risk of death from ovarian cancer in Taiwan: a country of low incidence of ovarian cancer.

This study was undertaken to examine whether there is an association between parity and age at first birth and risk of ovarian cancer. The study cohort consisted of all women with a record of a first and singleton childbirth in the Birth Register between 1978 and 1984. We tracked women from the time of their first childbirth to December 31, 2003, and their vital status was ascertained by linking records with the computerized mortality database. Cox proportional hazard regression models were used to estimate the relative risks (RR) of death from ovarian cancer associated with parity and age at first birth. There were 322 ovarian cancer deaths during 27,402,995.5 person-years of follow-up. The mortality rate of ovarian cancer was 1.18 cases per 100,000 person-years. A trend of increasing risk of ovarian cancer was seen with increasing age at first birth. The adjusted RR was 0.69 (95% CI = 0.52-0.90) for women who bore two children, and 0.30 (95% CI = 0.21-0.42) for women with three or more births, respectively, when compared with women who had given birth to only one child. There was a significant decreasing trend in the adjusted RR of ovarian cancer with increasing parity. This study provides evidence that parity may confer a protective effect on the risk of ovarian cancer.