What are the risk factors and impacts of anxiety in women and men in heterosexual couples undergoing IVF as part of a randomised trial, with a delay in embryo transfer in one arm? Duration of infertility, ethnicity, and male partner's anxiety levels were associated with women's anxiety at the start of treatment, while initial anxiety score, partner's anxiety score at embryo transfer, ethnicity, and clinic location were associated with women's anxiety levels at embryo transfer; although women undergoing IVF were more anxious than their partners for slightly different reasons, their self-reported state anxiety was not associated with achieving clinical pregnancy, nor with switching from delayed frozen embryo transfer to fresh embryo transfer in an IVF trial. Use of IVF treatment continues to rise and patients undergoing IVF are anxious. Participating in a randomised controlled trial (RCT) with uncertainty of arm randomisation might increase their anxiety, while a delay in treatment may add further to anxiety. A mixed methods study was conducted using data from the multi-centre E-Freeze RCT cohort conducted across 13 clinics in the UK from 2016 to 2019. A regression analysis on anxiety scores of couples undergoing the IVF trial and a qualitative analysis of participant questionnaires were performed. Six hundred and four couples participating in the E-Freeze trial, who had at least one useable State-Trait Anxiety Inventory (STAI) State Anxiety subscale (STAI-S) standardised self-report questionnaire for at least one of the partners, were included in the study. STAI-S scores were measured at consent for trial (T1) and again at embryo transfer (T2). Linear and log-binomial regression were used to explore the association between characteristics and STAI-S scores, and the associations between STAI-S scores and non-compliance and clinical pregnancy, respectively. Responses to the open text question were qualitatively analysed inductively using content analysis. Women's STAI-S scores at T1 (consent) were associated with their ethnicity, duration of infertility, and their male partner's STAI-S score at T1. Women's STAI-S scores at T2 (embryo transfer) were associated with their ethnicity, location of fertility clinic, their STAI-S score at consent, and their male partner's STAI-S score at embryo transfer. The adjusted coefficient (95% CI) for women's STAI-S scores at T2 was -4.75 (-7.29, -2.20, P < 0.001) for ethnic minority versus White, -2.87 (-4.85, -0.89, P = 0.005) for Scotland versus England, 0.47 (0.37, 0.56, P < 0.001) for each point increase in their own score at T1, and 0.30 (0.21, 0.40, P < 0.001) for each point increase in their male partner's score at T2. On average, women had higher STAI-S scores than men at both time points, and a larger increase of scores between the two time points. However, women's STAI-S scores were not associated with either non-compliance with trial allocation in the 'freeze-all' trial arm, or with chances of pregnancy. Both partners, but particularly women, described feeling anxious about the outcome of IVF, with women carrying the added worry of believing that feeling stressed might itself affect the outcome. Participants highlighted the important role of support from staff in helping them to manage their anxiety. Data were not available on education level or social support, which might influence anxiety scores. Men's baseline characteristics were not collected. Identifying couples at increased risk of emotional distress may be improved by using standardised anxiety measures at the start of the fertility treatment. Women can be reassured that their self-reported state anxiety does not affect their chances of achieving clinical pregnancy through IVF, and this may help to reduce anxiety levels. The psychological wellbeing and experiences of couples undergoing IVF could be supported by patient-centred care: making information about the whole process of treatment and choices available to both partners in accessible formats; ensuring interactions with staff are kind and supportive; and acknowledging and addressing the different concerns of women undergoing IVF and their partners. This study was an NIHR HTA (National Institute for Health and Care Research Health Technology Assessment) funded study. There are no conflicts of interest to declare. ISRCTN registry: ISRCTN61225414.