The goal of the randomised controlled study (RCT) is to identify novel strategies that can improve the luteal phase endocrinology after ovarian stimulation, with the aim of using less or no luteal support in IVF while making the endometrium thinner and more receptive to embryo implantation.

The high estradiol levels generated in IVF by multiple dominant follicles at the follicular-luteal phase transition generate powerful negative feedback at the pituitary gland, effectively switching off gonadotropin production. Without stimulation by endogenous luteinizing hormone (LH), the corpora lutea undergo involution and menstruation follows, preventing the establishment of pregnancy.

To address these detrimental effects of ovarian stimulation, the investigators will do a RCT with the use of aromatase inhibitors, which suppress the conversion of androgens to estrogens to ameliorate the unwanted effects of high estrogens.

This study is designed to test the hypothesis that preventing supra-physiological estrogen levels during ovarian stimulation restricts pituitary suppression sufficiently to allow LH levels to rise and support the corpora lutea, leading to a normal post-ovulatory luteal phase, evidenced by a normal duration and luteal phase endocrine profile. To test this, consenting oocyte donors will be randomized to receive co-treatment with aromatase inhibitors or no co-treatment during ovarian stimulation. Following oocyte retrieval, no luteal support will be provided to donors in both Groups.

Inclusion Criteria:

• Meet the clinical criteria for acceptance as oocyte donors (healthy women who volunteers to donate oocytes to other women).
• Regular ovulatory cycle of 26-32 days.
• Age: 18-35 years old.
• Written consent.

Exclusion Criteria:

• Contraindications for ovarian stimulation or aspiration of oocytes according to local guidelines
• Polycystic Ovary Syndrome (PCOS)
• Allergy towards study drug
• Women who have had a hormone or copper intra-uterine device (IUD) within 3 months

Exclusion of patients after earlier inclusion in the study in case of

• Patient withdrawal of consent
• Lack of compliance with medication
• Medical complication arising from IVF treatment that requires the cycle to be terminated
• Serious adverse events (SAE) or serious adverse reactions (SAR) including severe allergy to study drug.
• Specific ARs to study drug: severe degree of hot flushed, severe degree of nausea/vomiting, severe diarrhea, severe degree of muscle and joint pain.

In case of exclusion of a patient after earlier inclusion in the study, a new patient will be included.

Yes, but patients will have genitals examined.