BACKGROUND: ET is traditionally performed on either day 3 or day 5 depending on the availability of embryos for transfer. When a large cohort of embryos is available, day 5 ET allows for better embryo selection based on blastocyst morphological criteria. However, when few embryos are available, the decision becomes whether to put back all good-quality embryos on day 3 or wait until day 5 to transfer embryos that form a blastocyst. OBJECTIVE(S): To determine whether the pregnancy rate would improve in patients of poorer prognosis, those with fewer than four embryos, if all embryos were grown to day 5. MATERIALS AND METHOD(S): Before 2008, patients undergoing an autologous IVF cycle who had 3 or fewer good-quality embryos (8 cells of grade 1 or 2) available on day 3 had embryos transferred on day 3. If there were four or more good-quality embryos available on day 3, embryos were grown to day 5. After 2008, all embryos were grown to day 5 regardless of number available on day 3. Ninety-six cycles in 2008 that would have previously met day 3 ET criteria but were grown to day 5 using the new protocol were compared with 71 cycles in 2007 with day 3 ETs. Patients were separated into two age groups (≤35 and 36–40). Data for 2007 and 2008 were retrospectively analyzed for mean number of embryos transferred, clinical pregnancy rate, implantation rate (presence of fetal heart), and the number of cycles cancelled owing to failure to grow to day 5. RESULT(S): Significantly more embryos were transferred in 2007 in both patient age groups compared with in 2008. Pregnancy rates were increased in 2008 for all patients, although this was not statistically significant. Implantation rates were significantly improved in poorer prognosis patients regardless of age when all embryos were transferred on day 5. Cancellation rates were not significant between groups. CONCLUSION(S): Implantation rates increased when embryos were transferred on day 5 in patients with poorer prognosis. This may be due to more favorable uterine conditions and decreased uterine contractility. Using day 5 ET for all patients would be ideal in a program that has a good blastocyst growth rate to increase the implantation rate in poorer prognosis patients and decrease the number of embryos transferred to limit the incidence of high-order multiples.TableAge20072008<35 Years old: No. of ETs3557Cancellations11 Mean embryos transferred2.9a.2.0a. Clinical ongoing %48.561.5 Implantation rate, % FHB23bP<.001.44.2bP<.001.36–40 Years old: No. of ETs3639Cancellations15 Mean embryos transferred2.92.2 Clinical ongoing %27.841 Implantation rate, % FHB9.5bP<.001.27.1bP<.001.a .b P<.001. Open table in a new tab