For the pure natural cycle (PNC), ovulation will be detected by a combination of ultrasound monitoring the follicular growth and serial measurement of luteinizing hormone (LH), estradiol (E2) and progesterone (P4) levels, which is recognised to be the most accurate method of correctly identifying ovulation.~For the modified natural cycle (MNC), a bolus of rhCG 6500 IU subcutaneous (sc) will be administered when 1 dominant follicle reaches 17mm or above, after excluding previous LH surge. This is the first prospective study where the differences on the hormonal profile for ovulation will be studied.~HYPOTHESIS:~H0: The hormonal profile is not different between the PNC and the MNC H1: The hormonal profile is different between the PNC and MNC~Administration of hCG is a popular method for triggering ovulation since it may avoid the need to perform IUI or FET at weekends. However, it is an intervention compared with the detection of the spontaneous LH rise. Moreover, due to the presence of hCG receptors in the human endometrium, administration of hCG might interfere with endometrial receptivity. Previous publications evaluating pregnancy rates demonstrated that the use of hCG injection during the natural cycle reduces the pregnancy rates, in FET and IUI. It remains unclear what might be the mechanism to explain the lower pregnancy rates: a possible direct effect of hCG on the hCG receptors of the endometrium or a modification of the hormonal profile after the administration of the hCG, affecting the window of implantation (WOI).
Natural cycles are evaluated during fertility treatments for different therapeutic options, like insemination (IUI), time intercourse (TI) or frozen-thawed embryo transfers (FET). Two possible protocols can be used for natural cycles, with or without hCG trigger. Usually, studies including natural cycles consider both options as equivalent, not considering the possible bias that the exogenous hCG (human chorionic gonadotrophin) might have. In this prospective study, Investigators aim to evaluate the differences in the hormonal profile during the ovulation when it occurs spontaneously vs triggered with a bolus of hCG.