The retention of the embryo in the transfer catheter after embryo transfer (ET) during in vitro fertilization is a very common phenomenon, encountered by even the most experienced operators, and embryos retained in the transfer catheter or its sleeve require a repeat transfer. The exact mechanism of embryo retention has not been explained. Therefore, the present study aimed to investigate the mechanism of embryo retention in the catheter during embryo transfer by using a transparent uterus model equipped with pressure sensors and a video recorder. The results indicate that pressure changes in the uterine cavity during ET can influence the distribution of the transferred fluid containing the embryo. Under certain conditions, the transferred fluid can flow backward in the catheter, which may lead to retention of the embryo in the catheter.