The objective of this study was to compare ongoing pregnancy rates per started cycle between patients randomized at consultation to perform embryo replacement either on day-3 or on day-5 of in-vitro culture. Randomized controlled trial All patients <43 years of age in whom in-vitro fertilization (IVF) was indicated were allowed to participate in the study (day-3 group; 234 patients − day-5 group: 226 patients). Ovarian stimulation was performed either using gonadotrophin-releasing hormone (GnRH) antagonists/recombinant follicle stimulating hormone (rec-FSH) (day-3: 70.1% of patients, day-5: 72.6% of patients) or using the long GnRH agonist protocol/urinary gonadotropins (day-3: 29.9 % of patients, day-5: 27.4% of patients). The random decision to initiate a cycle leading to day-5 as compared to a day-3 transfer was associated with a significantly lower chance of embryo cryopreservation (day-3: 61.5%, day-5: 50.4%, p<0.02) (Table 1). Ongoing pregnancy rate per started cycle did not differ between the two groups compared (day-3: 32.1%-95%CI: 26.4–38.2; day-5: 33.2%-95%CI: 27.3–39.5). Advising patients at consultation to initiate an IVF cycle leading to a day-5 as compared to day-3 transfer does not appear to increase the probability of ongoing pregnancy while it is associated with a significantly lower probability to obtain cryopreserved embryos.