Menstrual cycle variability and the likelihood of achieving pregnancy.

The menstrual cycle is an important indicator of underlying hormonal function. Although menstrual cycle variability (sometimes referred to as 'regularity') is associated with a variety of demographic, behavioral, occupational, and environmental factors, as well as with several chronic diseases, few studies have examined its association with fecundity. We investigated whether a woman's menstrual cycle variability was associated with the likelihood of her achieving pregnancy. In this prospective study, we analyzed 3,536 menstrual cycles from 401 women (aged 19-41) recruited from 1990-1994. The women provided daily diaries recording menstrual bleeding, intercourse, and birth control use. Urine samples were assayed for human chorionic gonadotropin to identify early pregnancies during each menstrual cycle. Each woman's menstrual cycle variability was defined by the standard deviation of her cycle lengths during followup. The median follow-up was eight cycles. The outcome was her per-cycle probability of pregnancy. We found that women with high menstrual cycle variability had a reduced (51% lower) per cycle probability of pregnancy (fecundity ratio: 0.49; 95% confidence interval: 0.31, 0.77) compared with women with minimal variability. This relationship was independent of a woman's age and her mean cycle length. Thus, researchers and clinicians using menstrual cycle characteristics as indicators of endocrine or reproductive health should include measures of cycle variability in addition to the more commonly examined cycle length.