By Melissa Schorr

NEW YORK, Mar 02 (Reuters Health) - Women who have irregular menstrual periods throughout their lifetime may be at greater risk of breaking a hip after they go through menopause, researchers report.

Women with irregular periods may have lower levels of estrogen and progesterone circulating in their body, which could result in a loss of bone density, according to the research team from University of Minnesota School of Public Health in Minneapolis. Hip fracture, a sign of the bone-thinning disease osteoporosis, is one of the leading causes of disability and loss of independence in elderly women.

Dr. Kristen E. Anderson, a co-author of the report, told Reuters Health that irregular cycles are one more item on the list of risk factors for osteoporosis, which include slight frame, low calcium intake, smoking, advanced age and family history.

"This (the results of this study) might be another reason a woman might consult a doctor to see if she is at risk," Anderson said. "We don't want to scare people. It's not an enormous risk factor, but there was a connection."

Estrogen and progesterone are crucial hormones that play a role in maintaining bone density. Studies have shown that a drop in the levels of these hormones due to a total cessation of menstruation can erode bone density. However, little is known about whether slight fluctuations in the menstrual period over a woman's lifetime could have this effect as well.

In the study, the researchers looked at data from the Iowa Women's Health Study, which included 33,000 women aged 55 to 69 who were followed for an average of 9 years.

The investigators found that women who reported persistent irregularity in how often they had their cycle did not have an increased risk of hip fracture. Neither did women who reported great variability in how long their cycles typically lasted.

But, women who had those two factors combined had an 82% greater chance of breaking their hip.

Women who had an irregular cycle only occasionally, though, had no greater likelihood of fracturing a hip than women whose cycles were consistently on time, the authors report in the February issue of the American Journal of Epidemiology.

The researchers cite a recent clinical trial that found progesterone seemed to help prevent bone deterioration in women with irregular periods.

"Women who show such menstrual disruptions might benefit from exogenous hormones to reduce their risk of hip fracture," Anderson and colleagues conclude. However, more follow-up studies would need to be conducted before such a recommendation should be made, Anderson said.

Related MEDLINEplus Pages:

• Hip Injuries and Disorders
• Menstruation and Premenstrual Syndrome