Birthing age and ovarian cancer risk.

Giving birth confers on women some protection against ovarian cancer. A new study suggests that the later in life the last pregnancy happens, the better the protection.

Malcolm C. Pike of the University of Southern California in Los Angeles and his colleagues assessed 477 women who had ovarian cancer and 660 women without it. Women who had four or more full-term births were only one-third as likely to get ovarian cancer as were women who had never had a child, bolstering earlier findings (SN: 7/5/97, p. 7). Even incomplete pregnancies conferred significant protection. Some scientists hypothesize that more pregnancies correspond to fewer lifetime ovulations, less wear and tear on the ovaries, and hence a lower cancer risk.

But the new findings also suggest that women who stop having children earlier garner less protection. For example, women who had a child after age 35 had half the risk of ovarian cancer as did women who never gave birth, but mothers who last gave birth before age 25 were only 20 percent less likely to develop the cancer than were women who never gave birth.

High concentrations of the hormone progesterone during pregnancy "eliminate many of the ovarian cells that are at special risk of becoming ovarian cancer," Pike says. Some cells that begin the process of becoming malignant accumulate over time. Thus, he hypothesizes, the later they're jettisoned by pregnancy, the greater the protective effect. The report appears in the July Fertility and Sterility.--N.S.