Effect of retained embryos on pregnancy outcomes of in vitro fertilization: a matched retrospective cohort study

Objectives: To explore the incidence of retained embryos (REs) in embryo transfer (ET) cycles and its effects on pregnancy outcomes in women undergoing in vitro fertilization (IVF). Methods This was a matched retrospective cohort study involving 29,160 ET cycles conducted from March 2016 and February 2021, in which ET cycles without RE were matched to the RE group at a 2:1 ratio. Clinical pregnancy, implantation, miscarriage, and live birth rate were compared between RE and without-RE group. Results Our study showed that the overall incidence of REs was 0.33% (95/29,160). The RE rate was significantly associated with the operator who expelled the embryos from the catheter ( P  < 0.001), suggesting the operator’s technique is a crucial factor affecting the embryo retention. A total of 95 repeat ET cycles due to RE were included in the study group, and 190 ET cycles without RE were matched to the study group (1:2). There were no significant differences between the RE group and matched group in terms of the implantation rate (35.6% vs. 38.0%, P = 0.608), clinical pregnancy rate (47.4% vs. 54.7%, P = 0.240), biochemical pregnancy rate (5.3% vs. 4.7%, P = 0.846), miscarriage rate (11.1% vs. 9.6%, P = 0.781), ectopic pregnancy rate (2.2% vs. 1.9%, P = 1.000), or live birth rate (41.1% vs. 48.9%, P = 0.208). Conclusions Our findings demonstrated that immediate retransfer of REs does not significantly affect IVF outcomes, which may provide counselling information for patients when REs are discovered and ET is reattempted. Attention to detail and frequent assessment of operator’s technique may facilitate avoidance of embryo retention.