Rigorous exercise and heavy smoking may reduce a woman's risk of infertility and pelvic cavity tumors, Harvard doctors announced in a report published last week.

As part of an ongoing study of female infertility, the doctors also discovered that women with heavier and especially painful periods are more likely to develop endometriosis, the abnormal growth of tissue lining the uterine.

The study--conducted by Dr. Daniel W. Cramer, an assistant professor of gynecology and obstetrics, and his colleagues--compared 268 infertile woman with endometriosis to 3794 women who gave birth between 1981 and 1983.

"Women who smoke or exercise regularly reduce their estrogen levels which can influence whether or not they get the disease," Cramer said. "They may lighten their periods which stops the tissue from growing."

The results of the report were published in the most recent edition of the Journal of the American Medical Association.

Consistent Results

"The findings fit in with previous studies on estrogen levels. Women who smoke have slightly lower estrogen levels, and endometriosis is estrogen dependent," said Robert Balbieri, a clinical fellow in obstetrics and gynecology who also studies the disease.

Neither Balbieri nor Cramer advocate smoking as an effective means to prevent endometriosis. "I don't think women should start smoking just to avoid endometriosis. The benefits aren't that great," Balbieri said this week.

If smoking does prevent endometriosis, "it's probably working by destroying enzymes that produce estrogen," Cramer told the New York Times. "And that has been associated with other pregnancy-related diseases caused by smoking."

The researchers also determined which women were most vulnerable to endometriosis.

"Women with short cycles, 27 days or less, longer period of flow, and especially more painful periods, had more than double the risk of endometriosis compared to women with shorter flow and longer cycles," Cramer said.

Heavier periods "might lead to more fluid coming out of the fallopian tubes, and that's how the disease gets started," Cramer said.

Researchers estimate that 10 to 15 percent of all pre-menopausal and one third of all infertile women have endometriosis, Cramer said. In addition to infertility, the disease can cause chronic pelvic pain and abnormal growths on the ovaries.