Rationale: Intrauterine insemination (IUI) is the treatment of first choice for couples with unexplained and mild male factor infertility in many countries, but it is controversial whether ovarian stimulation improves fertility outcomes. In recent retrospectively collected data, investigators found that in couples with unexplained and mild male factor infertility undergoing IUI, ovarian stimulation with letrozole increased live birth rate as compared to natural cycle IUI without substantially increasing the multiple pregnancy rate. Investigators therefore perform a randomized clinical trial (RCT) on the subject in the Centre of Reproductive Medicine, Peking University Third Hospital, Beijing, China.

Objective: To test the hypothesis that in couples with unexplained or mild male factor infertility scheduled for an IUI program ovarian stimulation with letrozole increases the live birth rate as compared to natural cycle treatment.

Study design Randomized clinical trial.

Study population Women diagnosed with unexplained or mild male factor infertility scheduled for treatment with IUI.

Intervention Women will be randomized for ovarian stimulation with letrozole or to natural cycle IUI. In the group allocated to ovarian stimulation, women will receive oral tablets letrozole 5 mg daily from cycle day 3-5 for 5 days. Investigators will treat the couples for 3 cycles, with a time horizon of 4 months.

Main study parameters/endpoints Primary outcome is ongoing pregnancy leading to live birth. Secondary endpoints are clinical pregnancy, multiple pregnancy, miscarriage rates, pregnancy complications and patients' costs.

Nature and extent of the burden and risks associated with participation, benefit and group relatedness The strategies compared are already broadly applied in current practice. No additional risks are expected. There is no benefit for participants, but the results may benefit future subfertile couples.

Inclusion Criteria:

• Being diagnosed with unexplained or mild male subfertility
• At least one sided tubal patency, established according to local protocol
• Normal or mild impairment of semen quality defined as a TMSC of 3 million or more based on at least one semen analysis

Exclusion Criteria:

• Woman with double sided tubal pathology
• Women with irregular cycles, PCOS or other endocrine disorders
• Impaired semen quality: pre-wash TMSC <3 million.