Exogenous Progesterone as Ovulation Trigger

Ovulation is triggered by the sudden surge of gonadotropins, FSH and mainly LH, which occurs during the late follicular phase. This LH surge creates the environment for follicular eruption by increasing the activity of the proteolytic enzymes that weaken the ovarian wall leading to follicle rupture. However, the stimuli leading to the LH surge arises are not clear yet. The current scientific paradigm postulates that high levels of estradiol at the end of the follicular phase may, by positive feedback, stimulate the sudden release of gonadotropins. However, several factors seem to contradict this theory. Ovarian stimulation cycles with high doses of gonadotropins in high responders leads to high levels of estradiol in an early phase without incurring in ovulation until the late follicular phase. Thus, there may be a narrow window in the late follicular phase in which ovulation may occur. As such, the sustained high levels of estrogen in the late follicular phase could be responsible for the LH surge. Nevertheless, studies using high dose exogenous estradiol in late follicular phase showed that there was no impact on gonadotrophin surge or ovulation. On the other hand, studies based on ovarian stimulation cycles with letrozole, with much lowers serum estrogen levels, showed that the ovulatory peak occurs in the same way, even in the presence of very low or decreasing levels of estradiol. Interestingly, patients treated with letrozole may present higher levels of LH at peak. Hence, the role of estrogens in inducing the gonadotrophin peak is yet unclear.

Progesterone may eventually have a role in the physiological induction of ovulation. Long-standing studies have shown that an increase in serum progesterone levels precedes the LH peak and supports the action of the LH on the follicle rupture. Evidence has long shown that elevated serum progesterone levels during controlled ovarian stimulation are associated with a considerable risk of earlier ovulation. Likewise, previous studies have shown that the administration of intramuscular progesterone during the late follicular phase leads to the onset of an endogenous LH peak, both following controlled ovarian stimulation and in a natural cycle. Also supporting these theories is a new protocol of endometrial preparation for frozen embryo transfer presented by our research group. This strategy, in which, in an advanced follicular phase, after establishing adequate endometrial thickness on ultrasound, exogenous administration of vaginal micronized progesterone is started, simulating a second phase of the cycle, with endogenous production of estrogens and exogenous administration of progesterone. Curiously, in the first obstetric assessment the presence of an adnexal mass is frequently been noted, probably corresponding to a corpus luteum. If so, exogenous progestins may have acted as a possible trigger of ovulation.