The goal is to investigate whether endometrial flushing with Lipiodol enhances fertility. Therefore, anatomo-pathological, histochemical and transcriptional changes of the endometrium after Lipiodol bathing in a general population of healthy volunteers will be determined.

A single-centre, cross-over, open-label trial will be performed. Healthy volunteers, fulfilling the inclusion/exclusion criteria, will be asked to do blood samples and ultrasounds for the detection of the LH (luteinizing hormone) surge in a natural cycle. The intervention group will undergo endometrial flushing with Lipiodol between day 6 and 8 of the cycle, while the control group between day 6 and 8 of the cycle will have a mock catheter introduction without any Lipiodol flush. An endometrial biopsy will be performed 7 days after the LH peak in all the participants. Furthermore, all the participants will undergo one cycle with flushing and one cycle with introduction of a mock catheter but without flushing with any medication. The wash-out period between the 2 biopsies will be 3 months.

Inclusion Criteria:

• Women aged ≥18 and <36 years
• Regular cycle (25-35 days)
• Signed informed consent
• Participants can be included only once in the trial
• Contraceptive: barrier method only.

Exclusion Criteria:

• known iodine allergy
• Body mass index 30 or ≤18.5
• Smoking
• Previous diagnosis of PCO (Polycystic Ovary Syndrome) (>12 antral follicles in one ovary, Rotterdam criteria 2003)
• Previous diagnosis of endometriosis, uterine malformations or intrauterine pathologies (submucosal fibroids, endometrial polyps, and intrauterine adhesions)
• Systemic diseases such as thyroid dysfunction
• Breast-feeding or pregnancy within the last 6 months
• Intrauterine devices (IUD)
• History of recurrent miscarriage
• Known history of infertility
• Previous uterine surgery
• Pregnancy wish during the course of the study
• Women who have been previously enrolled in the trial
• Those unable to comprehend the investigational nature of the proposed study