New Strategies to Evaluate the Reproductive Tract Microbiome

The goal of this observational study is to evaluate alternative less invasive sampling methods for the analysis of the endometrial microbiome. For that, vaginal swab and endometrial fluid samples will be assessed in comparison with the standard sampling method (an endometrial biopsy), in women aged 18-50 years on an oocyte donation program and/or who attend the clinic for routine gynecological controls, and whose samples will be collected during the secretory phase of a natural menstrual cycle. The study aims to: * Evaluate the pontential of the less invasive techniques to assess the endometrial microbiome, compared to the endometrial biopsy, as well as their safety. * Evaluate the microbiome's stability in a period of 1 to 3 months. Participants will undergo a sampling round (of the 3 sampling methods), and then a second round 1, 2 or 3 months later, as assigned by the investigator.