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 to 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 rates were compared between the with-RE and without-RE groups. Results Our study showed that the overall incidence of REs was 0.33% (95/29,160). There was a statistically significant difference in RE rate among the operators (P < 0.001), suggesting that the embryo retention rate may be affected by the individual operator. A total of 95 repeated 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 and matched groups in terms of 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 The present findings demonstrated that immediate retransfer of REs did not significantly affect IVF outcomes, which may provide counselling information for patients when REs are identified and ET is reattempted. The incidence of REs was associated with the operator who expelled the embryos from the catheter. Attention to detail and frequent assessment of the operator's technique may facilitate avoidance of embryo retention.