OBJECTIVE What is the percentage of overtreatment, i.e. fertility treatment started too early, in couples with unexplained infertility who were eligible for tailored expectant management? CONCLUSIONS Overtreatment occurred in 36% of couples with unexplained infertility who were eligible for an expectant management of at least 6 months. BACKGROUND Prognostic models in reproductive medicine can help to identify infertile couples that would benefit from fertility treatment. In couples with unexplained infertility with a good chance of natural conception within 1 year, based on the Hunault prediction model, an expectant management of 6-12 months, as recommended in international fertility guidelines, prevents unnecessary treatment. METHODS A retrospective cohort study in 25 participating clinics, with follow-up of all couples who were seen for infertility in 2011-2012. METHODS In all, 9818 couples were seen for infertility in the participating clinics. Couples were eligible to participate if they were diagnosed with unexplained infertility and had a good prognosis of natural conception (>30%) within 1 year based on the Hunault prediction model. Data to assess overtreatment were collected from medical records. Multilevel regression analyses were performed to investigate associations of overtreatment with patient and clinic characteristics. RESULTS Five hundred and forty-four couples eligible for expectant management were included in this study. Among these, overtreatment, i.e. starting medically assisted reproduction within 6 months, occurred in 36%. The underlying quality indicators showed that in 34% no prognosis was calculated and that in 42% expectant management was not recommended. Finally, 16% of the couples for whom a correct recommendation of expectant management for at least 6 months was made, started treatment within 6 months anyway. Overtreatment was associated with childlessness, higher female age and a longer duration of infertility. No associations between overtreatment and clinic characteristics were found. CONCLUSIONS The response rate was low compared with other fertility studies. Evaluation of possible selection bias showed that responders had a higher socio-economic status than non-responders. CONCLUSIONS Our findings show that developing and publishing guideline recommendations on tailored expectant management (TEM) is not enough and that overtreatment still occurs frequently. Future research should focus on tailored efforts to implement guideline recommendations on TEM. BACKGROUND Supported by Netherlands Organisation for Health Research and Development (ZonMW). ZonMW had no role in designing the study, data collection, analysis and interpretation of data or writing of the report. Competing interests: none. BACKGROUND www.trialregister.nl NTR3405.