The association between weight, physical activity, and stress and variation in the length of the menstrual cycle.

The association between weight, physical activity, and stress and variation in the length of the menstrual cycle was prospectively examined in 166 college women, aged 17-19 years, who kept menstrual diaries during their freshman year. The unadjusted probability of a menstrual cycle being longer than 43 days was 5%. Women with a history of long cycles were more likely to have a long cycle during the study (odds ratio (OR) = 4.3). Stressors, characterized by situations which create a demand for performance or require adjustments to new demands, also increased the risk of a long cycle. Odds ratios for gain events and for coping with multiple performance demands (2 vs. 0) were 1.9. Starting college increased the risk of long cycles (OR = 2.3) regardless of whether a woman had left home. Moderate exercise minimally increased the probability of a long cycle (OR = 1.1, 75th vs. 50th percentiles). Change in weight (OR = 1.9) and being overweight (OR = 1.2, 15% above standard weight for height) were independently associated with the probability of long cycles. When 17- to 43-day cycles were evaluated, a history of long cycles lengthened expected cycle length by 1.42 days, while dieting, living on campus, and starting college tended to shorten expected length by 1.38, 0.90, and 0.64 days, respectively. Further investigation of the biologic mechanisms that mediate the stress effect is warranted.