This randomized trial was designed as non-inferiority trial aiming to compare ongoing pregnancy rates following LPS with 600 mg/day vs 800 mg/day vaginal VMP. All patients will undergo an artificial cycle frozen embryo transfer (AC-FET) with transdermal estradiol 6mg/day Patients undergoing an artificial cycle FET will start estrogen priming with transdermal estradiol 6mg/day (Estrogel®) on cycle D1-D3. Following 10-12 days of estrogen priming, patients will be randomized to luteal phase support with a standard formulation (200mg tid, Utrogestan®) or a new formulation (400mg bid) VMP. All patients will undergo a serum P measurement on the day before embryo transfer (ET). Patients with P<10 ng/ml will receive a supplement of oral micronized progesterone 300mg, while patients with P≥10ng/ml will maintain the previous luteal phase support (LPS) protocol

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).