Women who have seemingly normal menstrual periods may lose bone mass rapidly if they do not ovulate every month, according to a study published recently in the New England Journal of Medicine.

Lack of menstruation, which can occur in women who exercise strenuously or don't eat enough, has long been associated with weakened bones, making sufferers more vulnerable in later years to osteoporosis. But until now, doctors assumed that women who menstruated regularly also produced hormones that kept their bones healthy.

The new research concludes that women who do not ovulate, or release an egg, every cycle lose 4 percent of the bone in their spines annually, even though they menstruate as usual. "There is far more variability in the normal menstrual cycle than anyone has realized, largely because it has not been looked for," said study director Jerilynn C. Prior of the University of British Columbia in Vancouver.

Some premenstrual symptoms may accompany ovulation. Prior said women can keep track of ovulation by checking themselves for mild breast tenderness, a regular sign of the egg's release.

She said stress and being too thin may cause disrupted ovulation, and women who regularly miss ovulation may need to take progesterone supplements to preserve their bones.

Women can have menstrual cycles that are regular in length and flow but still not ovulate. In such cycles, women produce normal amounts of estrogen but less than normal progesterone.

In the study, doctors monitored the menstrual periods of 66 women ranging in age from 21 to 42. One third were runners preparing for a marathon, one third were recreational runners and the rest were normally active non-runners.

Researchers expected to see more menstrual disruptions in the marathoners. Instead they found that all the women had normal cycles 97 percent of the time, and ovulation disturbances occurred in 29 percent of their cycles. But the disturbances were no more common in the avid runners.

"These results suggest that the maintenance of peak bone density throughout adulthood requires normal ovarian production of both estrogen and progesterone," the researchers wrote. "It is possible that a substantial percentage of premenopausal women who have apparently normal menstrual cycles may instead have asymptomatic ovulatory disturbances."

Women who take birth control pills do not ovulate, but they are not susceptible to bone loss because the pills contain hormones, the study said.