STUDY QUESTION:How good is fertility knowledge and what are treatment beliefs in an international sample of men and women currently trying to conceive? SUMMARY ANSWER:The study population had a modest level of fertility knowledge and held positive and negative views of treatment. WHAT IS KNOWN ALREADY:Few studies have examined general fertility treatment attitudes but studies of specific interventions show that attitudes are related to characteristics of the patient, doctor and context. Further, research shows that fertility knowledge is poor. However, the majority of these studies have examined the prevalence of infertility, the optimal fertile period and/or age-related infertility in women, in university students and/or people from high-resource countries making it difficult to generalize findings. STUDY DESIGN, SIZE, DURATION:A cross-sectional sample completed the International Fertility Decision-making Study (IFDMS) over a 9-month period, online or via social research panels and in fertility clinics. PARTICIPANTS/MATERIALS, SETTING, METHODS:Participants were 10 045 people (8355 women, 1690 men) who were on average 31.8 years old, had been trying to conceive for 2.8 years with 53.9% university educated. From a total of 79 countries, sample size was >100 in 18 countries. All 79 countries were assigned to either a very high Human Development Index (VH HDI) or a not very high HDI (NVH HDI). The IFDMS was a 45-min, 64-item English survey translated into 12 languages. The inclusion criteria were the age between 18 and 50 years and currently trying to conceive for at least 6 months. Fertility knowledge was assessed using a 13-item correct/incorrect scale concerned with risk factors, misconceptions and basic fertility facts (range: 0-100% correct). Treatment beliefs were assessed with positive and negative statements about fertility treatment rated on a five-point agree/disagree response scale. MAIN RESULTS AND THE ROLE OF CHANCE:Average correct score for Fertility Knowledge was 56.9%, with greater knowledge significantly related to female gender, university education, paid employment, VH HDI and prior medical consultation for infertility (all P < 0.001). The mean agreement scores for treatment beliefs showed that agreement for positive items (safety, efficacy) was correlated with agreement for negative items (short/long-term physical/emotional effects) (P > 0.001). People who had given birth/fathered a child, been trying to conceive for less than 12 months, who had never consulted for a fertility problem and who lived in a country with an NVH HDI agreed less with negative beliefs. HDI, duration of trying to conceive and help-seeking were also correlates of higher positive beliefs, alongside younger age, living in an urban area and having stepchildren. Greater fertility knowledge was associated with stronger agreement on negative treatment beliefs items (P < 0.001) but was unrelated to positive treatment beliefs items. LIMITATIONS, REASONS FOR CAUTION:There was volunteer bias insofar as more women, people of higher education and people with fertility problems (i.e. met criteria for infertility, had consulted a medical doctor, had conceived with fertility treatment) participated and this was true in VH and NVH HDI countries. The bias may mean that people in this sample had better fertility knowledge and less favourable treatment beliefs than is the case in the general population. WIDER IMPLICATIONS OF THE FINDINGS:Educational interventions should be directed at improving knowledge of fertility health. Future prospective research should be aimed at investigating how fertility knowledge and treatment beliefs affect childbearing and help-seeking decision-making.