Cumulative Live Birth Rates After Cleavage-stage Versus Blastocyst-stage Embryo Transfer

All the participants will receive a long GnRH-agonist, ultra-long GnRH-agonist, short GnRH-agonist or GnRH antagonist protocol in combination with recombinant FSH. HCG will be administered for final oocyte maturation. Patients will have a single blastocyst-stage embryos transferred. The outcomes of all the embryo transfers within 1 year after randomization will be followed up. Single embryo transfer (SET) is required for the first 3 embryo transfers within 1 year after randomization. For embryo transfers beyond the third within the 1 year, patients' treatment must follow their randomized allocation, and SET is no longer mandatory. Luteal phase support will be administered before embryo transfer. If pregnancy is confirmed, luteal phase support will be continued until 10 weeks of gestation. The follow up will be continued until 6 weeks after delivery.

All the participants will receive a long GnRH-agonist, ultra-long GnRH-agonist, short GnRH-agonist or GnRH antagonist protocol in combination with recombinant FSH. HCG will be administered for final oocyte maturation. Patients will have a single cleavage-stage embryos transferred. The outcomes of all the embryo transfers within 1 year after randomization will be followed up. Single embryo transfer (SET) is required for the first 3 embryo transfers within 1 year after randomization. For embryo transfers beyond the third within the 1 year, patients' treatment must follow their randomized allocation, and SET is no longer mandatory. Luteal phase support will be administered before embryo transfer. If pregnancy is confirmed, luteal phase support will be continued until 10 weeks of gestation. The follow up will be continued until 6 weeks after delivery.