Elective cryopreservation of all day 5 blastocysts is more effective than using day 6 blastocysts for improving pregnancy outcome in stimulated cycles

Aim: To evaluate the efficacy of cryopreservation of all blastocysts for future transfers in stimulated cycles. Methods: We carried out fresh blastocyst transfer cycles on day 5 ( n = 290) or day 6 ( n = 119) and thawed blastocyst transfer cycles that were frozen on day 5 ( n = 136), day 6 ( n = 71) or day 6 electively ( n = 21). We retrospectively compared the clinical outcome of fresh blastocyst transfers with thawed blastocyst transfers according to the day of blastocyst transfer or freezing. Results: The clinical implantation rates in women with stimulated cycles were significantly higher after the transfer of thawed blastocysts compared with the transfer of fresh blastocysts (day 5, P < 0.0005; day 6, P < 0.00005). Although the implantation rate of fresh day 6 transfer cycles was lower than that of elective day 6 frozen–thawed cycles, this difference was not statistically significant ( P = 0.17). Conclusions: Thawed blastocysts demonstrated a better potential for implantation when compared with fresh blastocysts in stimulated cycles. We concluded that elective cryopreservation of all blastocysts on day 5 is an effective option to improve the clinical outcome in stimulated cycles. Additionally, with cryopreservation of all day 6 blastocysts, the implantation rates of first embryo transfers may increase by allowing the best‐quality blastocysts to be transferred in thawed cycles. (Reprod Med Biol 2008; 7 : 75–83)