This study aims to compare the efficacy, in terms of progesterone levels and clinical outcomes, of two different presentations of micronized vaginal progesterone (400 mg vs. 200 mg) used in endometrial preparation in artificial cycle (AC) for embryo transfer.

This is a non-inferiority, retrospective case-control study, with at least 200 cycles analyzed (at least 100 per arm) and the primary outcome is the serum progesterone level on the day of the embryo transfer.

Inclusion Criteria:

• Patients with primary or secondary infertility undergoing endometrial preparation for an embryo transfer using standard doses of oral or transdermal estrogens.
• Age between 18 and 50 years.
• Normal uterine cavity verified by imaging techniques.
• Serum progesterone documented on the day of embryo transfer.

Exclusion Criteria:

• Patients with a history of endometritis.
• Patients diagnosed with Asherman's syndrome.
• Patients with a different supplementation regimen or doses than those of the study groups.

The population will be selected among those patients who have undergone embryo transfer treatment with endometrial preparation in a artificial cycle at the Instituto Bernabeu.

The information collected in the study will always be treated as grouped data and never as individual or personal data, thus maintaining anonymity and confidentiality.