P-541 Does the IVF embryo remain at the site of deposition in successful IVF cycles?

Abstract Study question Does the placement of the IVF embryo differ from the implantation site? Summary answer At five weeks the implanted embryo has not moved in the lateral plane of the endometrial cavity but is slightly lower in the vertical plane. What is known already We previously showed that vertical placement of embryos approximately 9 mm from the fundus and equidistant from the tubal ostia has the highest implantation rate. Whether the embryo migrates from the point where the droplet is deposited before implantation is controversial. We used 3D ultrasound to compare the point of placement with the location of the gestational sac (GS) at 5 weeks gestational age (GA). Study design, size, duration Prospective analysis of 292 transfers of single egg donation blastocysts (2018-2020) and 139 resultant pregnancies in which the position of the GS in the coronal plane at 5 weeks was compared to that of the embryo flash immediately post transfer. A Voluson 6 with a 4D RAB4-8 transabdominal probe imaged the embryo transfer and a 4D RIC-5-9 transvaginal probe imaged the pregnancy scan (GE Healthcare, Austria). Participants/materials, setting, methods Consecutive patients with a normal uterine cavity undergoing egg donation in a single private IVF Center. Embryo transfers were performed under 2D US guidance using a soft Wallace Catheter. A 3D volume of the uterus was acquired immediately post-transfer. A second 3D scan was performed at 5 weeks GA. Distances of the embryo flash and the center of GS from the fundus, ostia and internal cervical os (ICO) were recorded and compared using SPSS. Main results and the role of chance Of 292 transfer procedures, 139 pregnancies who had the first ultrasound in our Center were included and analysed. The mean distance between the embryo flash and the uterine fundus was significantly less than the distance between the center of the GS and the uterine fundus [(9,49±3,03 vs 11,02±3,9 mm respectively (p < 0,05)]. The mean distances between the embryo flash and the right and left ostium were 16,66 ± 4,96 and 16,69 ± 4,61 mm respectively, with no significant differences from the same distances taken from the center of the GS. Limitations, reasons for caution As the image is of the embryo flash and not the embryo, this might have been misleading. Wider implications of the findings These findings extend and support the proposal of an optimal point of embryo placement for pregnancy. The apparent vertical difference in the implantation of the embryo could be due to uterine growth or it could be consequence of uterine peristalsis. This bears further study. Trial registration number not applicable