The influence of age at menarche, menstrual cycle length and bleeding duration on time to pregnancy: a large prospective cohort study among rural Chinese women.

ABSTRACT

OBJECTIVE: To investigate the relations among age at menarche (AAM), menstrual cycle length, menstrual bleeding duration and time to pregnancy in a large cohort of rural Chinese women. DESIGN: A prospective cohort study. SETTING: Local family-planning service agencies and maternal/child care service centres. POPULATION A total of 391 320 rural women of reproductive age who participated in the National Free Pre-pregnancy Checkups and were planning to conceive were enrolled. METHODS: Menstrual characteristics were collected via face-to-face interviews. The Cox proportional hazards model were used to estimate the fecundability ratios (FRs) and 95% confidence intervals for each measure relative to its reference category. MAIN OUTCOME MEASURES: Time to pregnancy. RESULTS: Women with an AAM later than 14 years of age were less likely become pregnant compared with women with AAM at 13-14 years of age (FR 0.93, 95% CI 0.92-0.94). Those with menstrual cycle lengths >29 days were less likely to come pregnant (FR 0.91, 95% CI 0.90-0.92) compared with the reference cycle length of 27-29 days. Women with bleeding durations of <4 (FR 0.88; 95% CI 0.86-0.91) or >5 days (FR 0.91; 95% CI 0.90-0.91) showed lower FRs compared with those reporting 4-5 days of bleeding. The associations were independent of maternal age, ethnicity, education level, occupation, tobacco use, alcohol use and body mass index. CONCLUSION: A later onset of menarche, longer menstrual cycle length, both shorter (<4 days) and longer (>5 days) bleeding duration were associated with a lower FR and longer time to pregnancy in rural Chinese women. TWEETABLE ABSTRACT A later menarche, longer cycle, shorter or longer bleeding duration were associated with lower fecundity.