Another difference among both protocols consists of the sudden blockage of endogenous LH secretion caused by the administration of the GnRH antagonist, which is usually given when the follicles reach the size of 12 mm (around day 6 of the menstrual cycle). At that developmental stage, the LH receptor is present in the follicles participating in follicular function. In women treated with the long protocol, low endogenous levels of LH have been associated with low pregnancy rates. At present, conflicting data have been reported with regard to the effect of either low or high levels of LH during the midfollicular phase on the pregnancy rates, regardless whether GnRH agonists or GnRH antagonists were used. However, all these studies were performed with recombinant FSH lacking all LH activity and one single day measurement of LH concentration in the serum may not reflect the endocrine activity of LH throughout follicular development. The need for the presence of LH during follicular development has been demonstrated unequivocally in women suffering of hypogonadotropic ovarian failure (WHO I), which lead to the development of Pergoveris® 150 I.U./75 I.U.