Introduction: Decision-making on embryo disposition is a source of distress and is subject to change over time. This paper analyses the willingness of couples undergoing in vitro fertilization to donate cryopreserved embryos for research from 15 days after embryo transfer to 12 months later, taking into account the influence of psychosocial, demographic, and reproductive factors. Materials and methods: Prospective longitudinal study, with 74 heterosexual couples undergoing in vitro fertilization in a public fertility centre in Portugal, recruited between 2011 and 2012. Participants were evaluated twice: 15 days after embryo transfer and 12 months later. Results: A significant decrease in patients' willingness to donate embryos for research over time was observed [86.5% to 73.6%; relative risk (RR) = 0.85; 95% CI 0.76-0.95]. A higher education level (>12 years) [adjusted RR (RRadj) = 0.79; 95% CI 0.64-0.96], considering research on human embryos to be important (vs. very important) (RRadj = 0.59; 95% CI 0.39-0.85) and practicing a religion less than once a month (vs. at least once a month) (RRadj = 0.73; 95% CI 0.53-1.00) seemed associated with unwillingness to donate embryos for research over time. Change towards non-donation happened mainly among couples who first considered that it was better to donate than wasting the embryos. Change towards donation occurred mostly among those stating that their priority at time 1 was to have a baby and who became pregnant in the meantime. Conclusions: Quality of care guided by patients' characteristics, values, preferences, and needs calls for considering the factors and reasons underlying couples' willingness to donate embryos for research over time as a topic in psychosocial guidelines for infertility and medically assisted reproductive care. ; The authors thank Sandra Sousa for her collaboration in data collection. This study was funded by FEDER from the Operational Program Factors of Competitiveness – COMPETE and by national funding from the FCT – Foundation for Science and Technology (Portuguese Ministry of Education and Science) within the project “Health, governance and accountability in embryo research: couples’ decisions about the fates of embryos” (FCOMP-01-0124-FEDER014453), and the Grants SFRH/BD/75807/2011 (to CS) and SFRH/BPD/80530/2011 (to CF), co-funded by the POPH Program, the IF/00956/2013 (to SS) and the IF/ 00829/2013 (to HM).