Multicenter prospective observational cohort study, comparing ongoing pregnancy rates in natural cycles for euploid embryo transfer in patients who present endometrial compaction at the time of the transfer versus those who have a stable or greater endometrial thickness with respect to the estrogenic phase.

Multicenter prospective observational cohort study. Based on our sample size calculation, the study group will consist of 206 women (exposed or "compaction" group: 103 women with a decrease of ≥ 5% in endometrial thickness between the estrogenic phase and the day of embryo transfer; non-exposed "non-compaction" group: 103 women with similar or greater endometrial thickness between these time points). The main objective of this study is to compare the ongoing pregnancy rates in natural cycles for euploid embryo transfer in patients who present endometrial compaction at the time of transfer versus those who with a stable or greater endometrial thickness with respect to the estrogenic phase. The estimated duration of the study is 24-30 months. Inclusion criteria are: 18 to 50 years of age, with primary or secondary infertility, subjected to endometrial preparation in a modified natural cycle for transfer of a genetically euploid blastocyst, from their own oocyte or oocyte donation, with a normal uterine cavity. Exclusion criteria are: uterine or endometrial disease (e.g., multiple myomatosis, severe adenomyosis, Asherman syndrome, refractory endometrium), conditions that prevent correct ultrasound assessment (tilted uterus), or a history of recurrent implantation failure or repeated miscarriages.

Inclusion Criteria:

• Aged 18 to 50 years
• Primary or secondary infertility
• Normal uterine cavity
• Endometrial preparation in a modified natural cycle for single embryo transfer
• Embryo in blastocyst state from own oocyte or oocyte donation cycles, who had normal results on preimplantation genetic testing for aneuploidy (PGT-A) via trophectoderm biopsy.

Exclusion Criteria:

• Uterine or endometrial disease (multiple myomatosis [>3 fibroids of > 3 cm], adenomyosis, Asherman syndrome)
• Difficulties in correctly measuring endometrial thickness due to a retroverted or tilted uterus
• History of recurrent implantation failure (3 or more transferred blastocysts of good quality, from their own oocyte [<35 years] or oocyte donation); recurrent early Pregnancy Loss (the loss of two or more pregnancies before 10 weeks of gestational age.
• Suboptimal endometrial response (endometrium < 6 mm on the day of ovulation triggering).

Women undergoing fertility treatment.

206 Women between 18-50 years, with primary or secondary infertility, undergoing fertility treatment that includes endometrial preparation in a modified natural cycle for transfer of previously analyzed frozen and euploid embryos.