Influence of medical conditions and lifestyle factors on the menstrual cycle.

BACKGROUND: Few studies have described medical and lifestyle factors associated with various menstrual cycle characteristics.
METHODS: We analyzed cross-sectional data collected from 3941 premenopausal women from Iowa or North Carolina participating in the Agricultural Health Study between 1994 and 1996. Eligible women were age 21-40, not taking oral contraceptives, and not currently pregnant or breast feeding. We examined four menstrual cycle patterns: short cycles (24 days or less), long cycles (36 days or more), irregular cycles, and intermenstrual bleeding.
RESULTS: Long and irregular cycles were less common with advancing age and more common with menarche after age 14, with depression, and with increasing body mass index. The adjusted odds of long cycles increased with increasing body mass index, reaching 5.4 (95% confidence interval [CI] = 2.1-13.7) among women with body mass indexes of 35 or higher compared with the reference category (body mass index of 22-23). Smoking was associated with short cycles. Long cycles, irregular cycles, and intermenstrual bleeding were associated with a history of infertility. Having long cycles was associated with a doubling in the adjusted odds of having a fetal loss among women who had been pregnant within the last 5 years (odds ratio = 2.3; 95% CI = 0.9-5.7).
CONCLUSIONS: Menstrual patterns are influenced by a number of host and environmental characteristics. Factors that perturb menstruation may increase a woman's risk of other reproductive disorders.