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Purpose: Variability in menstrual cycle length, largely determined by variation in follicular phase length, is related to several health outcomes, yet the causes of this variability are incompletely understood. We sought to identify characteristics associated with follicular phase length.

Methods: We used the North Carolina Early Pregnancy Study to describe factors correlated with timing of ovulation (follicular phase length). Women collected daily urine specimens and recorded vaginal bleeding. Specimens were assayed for estrone 3-glucuronide and pregnanediol 3-glucuronide, which in turn were used to estimate the day of ovulation. All other variables were assessed through interview. Associations with follicular phase length were evaluated using a multiple regression model.

Results: We determined follicular phase length for the first cycles of 201 women. Women with a history of miscarriage tended to have shorter follicular phases (2.2 days). Longer duration of oral contraceptive (OC) use and recent OC use (in the last 90 days) were both correlated with longer follicular phase. Occasional marijuana users (up to three times in the last 3 months) had a longer follicular phase than nonusers (3.5 days); the follicular phase in frequent users (more than three times) was almost 2 days longer than that of nonusers.

Conclusions: The association between marijuana use and longer follicular phase is consistent with prior rhesus monkey research that shows ovulatory delay or inhibition.