This study is designed to collect information about the characteristics of endometrial fluid at day 2 and day 4 post-ovulation in normal fertile women.

Implantation rates have stabilised around 35% in IVF and a current key rate limiting step is achieving successful implantation of the embryo. 1/3 of implantation failures are estimated to be caused by a problem with the receptivity of the endometrium. In recent years, several measurable parameters have been reported to assess endometrial receptivity. However, one parameter remains undescribed, the endometrial fluid viscosity.

It is generally believed that the viscosity change post ovulation is influenced by the luteal phase endocrine milieu, and it is possible that the viscosity of the endometrial fluid has an impact on implantation.

Inclusion Criteria:

• Age: ≤ 35 years
• Women eligible and qualified for egg donation at IVIRMA Bilbao
• Proven fertile (having given birth to at least one spontaneously conceived normal child)
• Signed written consent including sharing of anonymized data

Exclusion Criteria:

• Contraindications for egg donation according to IVIRMA Bilbao guidelines

Women eligible and qualified as egg donors at IVIRMA Bilbao that meets the inclusion criteria