The aim of this RCT is to compare differences in the efficacy and safety between cleavage-stage embryo transfer and blastocyst-stage embryo transfer in IVF/ICSI treatment cycle, taking into account of subsequent vitrified embryo transfers. Subjects with 3 or more transferrable cleavage embryos will be randomized to the cleavage-stage or blastocyst-stage embryos transfer group. The primary outcome is cumulative live birth rate (CLBR) per patient until the first live birth from one initiated oocyte retrieval cycle, calculated using outcomes from the first three embryo transfers within 1 year after randomization.

This is a multicenter, randomized clinical trial comparing the efficacy and safety between cleavage-stage embryo transfer and blastocyst-stage embryo transfer in IVF/ICSI treatment cycle, taking into account of subsequent vitrified embryo transfers.Randomization will be performed on day 2/3 after oocyte retrieval, when at least 3 embryos are achieved. Patients in group A will have 1 cleavage-stage embryo transferred. Patients in group B will have 1 blastocyst-stage embryo transferred. The outcomes from all the embryo transfers within 1 year after randomization will be followed up. If a pregnancy/live birth is not achieved, single embryo transfer is required for the first 3 embryo transfers within 1 year after randomization. For embryo transfers beyond the third within the 1 year, patient's treatment must follow their randomized allocation, and SET is no longer mandatory. The follow-up period is 2 years from the day of randomization.Due to the COVID-19 pandemic, the participants who were unable to undergo the embryo transfers in 1 year of randomization will have 3 months extension for frozen embryo transfer. The follow-up for these participants will be extended for 3 months as well.

Inclusion criteria:

1. Women aged ≥20 and ≤40 years.
2. Women with the number of transferrable cleavage embryos ≥ 3;
3. Women undergoing their first or second cycle of IVF or ICSI.

Exclusion criteria

1. Women who have been diagnosed with uterine abnormalities (confirmed by three-dimensional ultrasonography or hysteroscopy), including malformed uterus (uterus unicornis, septate uterus or duplex uterus), submucous myoma, or intrauterine adhesion
2. Women who plan to undergo In Vitro Maturation (IVM);
3. Women who plan to undergo Preimplantation Genetic Diagnosis (PGD) /Preimplantation Genetic Screening (PGS).
4. Women who have Women who have hydrosalpinx visible on ultrasound.
5. Women who have experienced recurrent spontaneous abortions, defined as 2 or more previous pregnancy losses.
6. Women who have been developed a "freeze-all" treatment plan for purpose of subsequent surgery, such as salpingectomy due to hydrosalpinx after oocytes retrieval.
7. Women with contraindications to assisted reproductive technology and/or pregnancy, such as uncontrolled hypertension, symptomatic heart diseases, uncontrolled diabetes, undiagnosed liver disease or dysfunction (based on serum liver enzyme test results), undiagnosed renal disease or abnormal renal function, severe anemia, history of deep venous thrombosis, pulmonary embolus or cerebrovascular accident, history of or suspicious for cancer, undiagnosed vaginal bleeding.