Objectives: To compare the implantation rate of embryos after 3 and 5 days of IVF culture. Design: Prospective randomization of ET depending on the weekday of ovum pick-up (OPU). Setting: University Department of Endocrinology and Reproduction. Patients: All women entering an outclinic IVF program. Interventions: Two hundred thirty-three ETs performed on day 3 after OPU and 410 performed on day 5 were analyzed. When blastocysts with a clear inner cell mass were available, a maximum of two were replaced. Results: On day 3 after OPU, 60 pregnancies per 233 ET (26%) and on day 5, 102 pregnancies per 410 ET (25%) were induced. The average implantation rate per embryo was 13% and 12%, respectively. After subdivision according to embryo morphology, pregnancy rate per ET (n = 59) and implantation rate per embryo on day 3 with exclusively unfragmented embryos were 32% and 18%, respectively, not significantly different from ET (n = 73) exclusively with embryos containing >0% and <20% fragments: 27% and 12%. After transfer on day 5, when one or more cavitating embryos were available (n = 227), pregnancy and implantation rates were 40% and 23%, statistically different from ET on day 3. On day 5, ET exclusively with morula stages showing signs of starting blastulation (n = 26), pregnancy rate and implantation rate were 12% and 11%, respectively, from ET (n = 157) with embryos not reaching the latter stage: 6% and 3%. Conclusions: Overall ET results after 3 and 5 days are comparable. After 5 days of culture, one to two embryos can be replaced with an average implantation rate of >23% per embryo, minimizing the incidence of triplets.
