REPRODUCTIVE FACTORS AND INCIDENCE OF EPITHELIAL OVARIAN-CANCER - A NORWEGIAN PROSPECTIVE-STUDY

To examine relations between the risk of epithelial ovarian cancer and time-related effects of pregnancies, we analyzed data from a prospect ive study of 1,145,076 women in Norway aged 20 to 56 years. The mean f ollow-up time per woman was 16.4 years and a total of 1,694 women were diagnosed with epithelial ovarian cancer. Incidence rate ratios (IRR) were estimated by Poisson regression analysis of person-years at risk . The risk of epithelial ovarian cancer decreased with an increasing n umber of full-term pregnancies (IRR = 0.56, 95 percent confidence inte rval = 0.48-0.67 for three pregnancies cf one). However, no further re duction in risk was seen after the third pregnancy. The association wi th parity became weaker with increasing age at last birth. Further, th e reduction in risk among parous women compared with nulliparous women was more pronounced shortly after birth. Both higher age at last birt h and shorter time since last birth were associated with a reduction i n risk, although these relations were seen mainly for the first and se cond births. Increasing age at first birth was associated with a decre ase in risk among uniparous women but not among multiparous women. Our results indicate that the relations between the incidence of epitheli al ovarian cancer and reproductive factors are more complex than previ ously believed.