Study question: What is the effectiveness of continued treatment with clomiphene citrate (CC) in women with World Health Organization (WHO) type II anovulation who have had at least six ovulatory cycles with CC but did not conceive?

Summary answer: When women continued CC after six treatment cycles, the cumulative incidence rate of the ongoing pregnancy rate was 54% (95% CI 37-78%) for cycles 7-12.

What is known already: If women with WHO type II anovulation fail to conceive with CC within six ovulatory cycles, guidelines advise switching to gonadotrophins, which have a high risk of multiple gestation and are expensive. It is however not clear what success rate could be achieved by continued treatment with CC.

Study design, size, duration: We performed a retrospective cohort study of women with WHO II anovulation who visited the fertility clinics of five hospitals in the Netherlands between 1994 and 2010. We included women treated with CC who had had at least six ovulatory cycles without successful conception (n = 114) after which CC was continued using dosages varying from 50 to 150 mg per day for 5 days.

Participants/materials, setting, methods: Follow-up was a total of 12 treatment cycles. Primary outcome was the cumulative incidence rate of an ongoing pregnancy at the end of treatment.

Main results and the role of chance: We recruited 114 women that had ovulated on CC for at least six cycles but had not conceived. Of these 114 women, 35 (31%) had an ongoing pregnancy resulting in a cumulative incidence rate of an ongoing pregnancy of 54% after 7-12 treatment cycles with CC.

Limitations, reasons for caution: Limitations of our study are its retrospective approach.

Wider implications of the findings: Randomized trials comparing continued treatment with CC with the relatively established second line treatment with gonadotrophins are justified. In the meantime, we suggest to only begin this less convenient and more expensive treatment for women who do not conceive after 12 ovulatory cycles with CC.

Study funding/competing interests: None.

Trial registration number: Not applicable.