MIcronized PROgesterone in Frozen Embryo Transfer Cycles

Inclusion Criteria: * Endometrial preparation with hormone replacement therapy * Age 18-43 years following an autologous IVF cycle (with or without preimplantation genetic testing for aneuploidy) * Age \< 50 years following an egg donation cycle * BMI \> 18 and \< 30 kg/m2 * blastocyst embryo transfer * Willing to participate in the study * Able to come to the Center to comply with the procedures of the study: blood tests, appointments and drug dispensation. Exlusion Criteria: * • Uterine diseases (e.g. submucosal fibroids, polyps, previously diagnosed Müllerian abnormalities) * Hydrosalpinx * Recurrent pregnancy loss (≥ 3 previous miscarriages) * Recurrent implantation failure (≥ 3 previously failed embryo transfers of good-quality blastocysts) * Allergy to study medication * Pregnancy or lactation * Contraindication for hormonal treatment * Personalized initiation of exogenous progesterone according to a previous endometrial receptivity assay test * Recent history of severe disease requiring regular treatment (clinically significant concurrent medical condition that could compromise subject safety or interfere with the trial assessment).