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Menstrual cycle characteristics are associated with fecundability

In this prospective time-to-pregnancy study of rural Chinese women, Zhang et al. sought to determine the association between age at menarche, menstrual cycle length and bleeding duration on time to pregnancy. While these associations have been examined in smaller or retrospective cohorts or in Western populations, this study is very large, prospective and of Chinese women. Chinese women have a later onset of menarche and fewer leiomyoma than do American Black and Caucasian women. Zhang et al. found that in this population of rural Chinese women, menarche at age 13 or 14, menstrual cycle length of 25–29 days and duration of bleeding of 4–5 days were associated with highest fecundability. While other cohorts have also found that menstrual cycles of less than 25 days were associated with reduced fecundability, ‘longer’ (30–34-day cycles) were not associated with reduced fecundability. In fact, Wise et al. found that women with a menstrual cycle length of 32–33 days had the highest fecundability (Wise et al. Am J Epidemiol 2011;174:701–9). While other studies have also suggested that later age at menarche and longer bleeding time are associated with reduced fecundability, this cohort was of a sufficiently large sample size to show a statistically significant association. This study does have some limitations. First, they did not assess duration of attempt to conceive at study entry. However, it is unlikely that this differs by any of the exposures. Second, while it is understandable that they excluded women with irregular menstrual cycles, it may be that age at menarche and menstrual cycle regularity may be controlled by a similar mechanism, such as hypothalamic function. An association between early onset of menarche and polycystic ovarian syndrome, a common cause of menstrual cycle irregularity, has been observed. Finally, they excluded women who stopped trying to conceive or started fertility medication. This may exclude the least fecund women. As they used a Cox model, an option would have been just to censor them at initiation of medication. Unfortunately they also did not include potential confounders – recent use of hormonal birth control and gravidity – in their models. Despite these study limitations, the study findings combined with the existing literature would suggest that women with longer menstrual cycles are less likely to conceive. This could be due to an increased rate of anovulation in these women or inappropriately timed or infrequent intercourse patterns. The mechanism by which delayed menarche could lead to reduced fecundability is not known. It is possible that both are due to underlying hypothalamic dysfunction. Short and long menstrual bleeding are also associated with reduced fecundability. Longer duration of menstrual flow may reflect structural abnormalities such as polyps or fibroids or anovulatory cycles, while a shorter duration may reflect inadequate endometrial proliferation. Future studies should explore potential causal mechanisms for these observed associations.