Menstrual cycle length in reproductive age women is an indicator of oocyte quality and a candidate marker of ovarian reserve.

Objective: The menstrual cycle is a finely tuned biological process comprising a precisely orchestrated sequence of events: follicular growth, selection and ovulation, extensive endometrial changes, corpus luteum (CL) growth and maturation, and luteolysis. Differences in the length of the menstrual cycle (MCL) have been associated with variable female fecundity. However, the reason for these differences is so far unknown. The donor-recipient model, separating uterine from ovarian factors, allows clarifying the origin of MCL-associated fecundity variations.

Study Design: We analyzed retrospectively 2015 oocyte donation cycles, resulting in 3427 embryo transfers (ET) and pregnancy follow-up.

Results: Surprisingly, we found that oocyte donors MCL of 34-35 days were strongly associated with significantly higher biochemical, clinical and ongoing pregnancy rates in woman who received the embryos, compared to the reference group of MCL of 27-29 days. Moreover, donors with longer MCL presented higher ovarian response to stimulation and lower amount of hormonal stimulation needed to achieve multifollicular growth. Conversely, MCL of <25 days were associated with a poorer ovarian response to stimulation, less cumulus oocyte complexes (COCs) and less mature oocytes (MII) retrieved; however, the quality of oocytes in these women is not associated to their ovarian response, as evidenced by the pregnancy rates obtained when transferred into an adequately prepared endometrium.

Conclusions: We conclude that oocyte quality, rather than natural endometrial preparation, is the main reason for the reported higher fecundity of women with longer MCL. This result is further confirmed by our data on bleeding length in the donor pool. Response to ovarian stimulation is the definitive test of ovarian reserve; moreover, since different MCLs result from varying length of the follicular phase, longer MCL should be associated with a higher number of follicular recruitment events. We hypothesize that MCL is associated with - and a marker of - ovarian reserve in healthy reproductive age women.