Inclusion Criteria:

• Primary or secondary infertility for at least a period of 1 year Participants whose previous pregnancy was achieved through MOH-IUI or IVF are eligible to join the study, provided they are willing to continue expectant management for at least six months before considering any further fertility interventions

• Diagnosis of UI after infertility work-up, i.e.

  • regular menstrual cycle ranging between 21-35 days,
  • total motile sperm count ≥ 10 million/ml,
  • no risk of tubal pathology (or in case of increased risk, tubal pathology uitgesloten middels tubal patency testing)

• A Hunault prognostic score ≥ 30% (calculated using the Hunault prediction model7, based on key prognostic factors including female age, subfertility duration, type of subfertility (primary or secondary), sperm motility and referral status),

• Assignment to expectant management during at least six months, in accordance with Dutch NVOG Guidelines (4).

• Female age ≥18 years old Female

• BMI <45 kg/m2

Exclusion Criteria:

• Uncorrected uterine factors, such as endometrial polyps or submucosal fibroids,
• Insufficient knowledge or understanding of the Dutch or English language and not willing or able to receive study information via a certified translator
• Not able or willing to provide (written) informed consent
• Contraindications for vaginal progesterone, in particular: females with allergy to peanuts or soya.
• Formal diagnosis of endometriosis