Funding for this research was provided by:
National Natural Science Foundation of China (81671520)
Received: 21 December 2020
Accepted: 15 April 2021
First Online: 23 April 2021
: Human Ethics Committee approval for the retrospective trial of frozen embryo transfer cycles came from the Institutional Review Board (IRB) of the Shanghai Ninth People’s Hospital on August 20, 2020 (Number: SH9H-2020-T268–1).
: All patients consent the data used for research and publication.
: The authors have nothing to declare.