Roles of FSH and LH during the follicular phase: insight into natural cycle IVF

A mounting interest in natural cycle IVF has challenged the medical community to better understand the mechanisms controlling the follicular phase and ovulation in particular, in an effort to optimize this procedure and its outcome. For practical reasons, the advancement of the follicular phase in the menstrual cycle is commonly timed according to the onset of last menses. However, this precludes knowing when the follicular phase truly begins and hampers the possibility of optimizing timing of late follicular-phase events, notably, the triggering of ovulation. Clinicians, therefore, use surrogate markers of follicular maturation, such as oestrogen production and follicular size. Because it is impossible to identify the low-amplitude intercycle basal FSH signal, efforts have reverted toward controlling when it takes place, either with exogenous oestrogen or with oral contraceptives. In the late follicular phase, the occurrence of LH surge results from a balance between the opposite effects of rising oestrogen concentrations, which favour the LH surge, and the opposing effects mediated by the gonadotrophin surge-attenuating factor, a peptide of ovarian origin. This review looks into the mechanisms that control these two hinges of the follicular phase, the basal FSH signal and LH surge, in the context of optimizing natural cycle IVF.