Information regarding the optimal timing for frozen-thawed embryo transfer (FET) following a stimulated in vitro fertilization (IVF) is lacking. One option is to perform FET in the first cycle following the stimulated IVF cycle, i.e. immediate transfer. Another option is to postpone FET for at least one menstrual cycle, i.e. delayed transfer. This randomized study aims to compare the ongoing pregnancy rate of immediate versus delayed FET following a stimulated IVF cycle.

Inclusion Criteria:

• Women aged ≤43 years at the time of IVF/ICSI treatment Undergoing IVF with a standard stimulation At least one frozen embryo or blastocyst The first FET cycle following ovarian stimulation in IVF/ICSI

Exclusion Criteria:

• Use of mild stimulation or natural cycle for IVF/ICSI treatment Severe ovarian hyperstimulation syndrome during IVF/ICSI treatment Preimplantation genetic diagnosis treatment Use of donor oocytes Presence of hydrosalpinx which is not surgically treated or endometrial polyp on scanning during ovarian stimulation

Individual participant data that underlie the results after deidentification (text, tables, figures, and appendices) and study protocol will be shared. Data will be available when beginning 3 months and ending 5 years following article publication. To achieve aims in the approved proposal, researchers who provide a methodologically sound proposal will be shared with.Data will be made available by the following way. Proposals should be directed to lihe198900@163.com. And data are available for 5 years at a third party website (link to be included after the article publication).

No masking because of the nature of the intervention