Assessment of the embryo flash position and migration with 3D ultrasound within 60 min of embryo transfer

Study question: Does the air bubble (embryo flash) position and migration as visualized with 3D ultrasound (US) within 60 min of embryo transfer correlate with clinical outcome following fresh ART transfer cycles?

Summary answer: The location of the embryo flash and the direction of its movement at 60 min, but not at 1 or 5 min after transfer, are associated with clinical pregnancy.

What is known already: Studies assessing the relation between the pregnancy rate and the position of the catheter tip and/or the position of the air bubbles following embryo transfer show conflicting results to date.

Study design, size and duration: This was a prospective cohort study including 277 infertile women undergoing ART between July 2011 and August 2013.

Participants/materials, setting and methods: Good prognosis patients undergoing fresh ART cycles within a single tertiary University unit were assessed by 3D US at 1, 5 and 60 min after embryo transfer. The distance of the embryo flash from the fundus was measured at these time points, along with the direction of the embryo flash movement within 60 min of transfer.

Main results and the role of chance: Within 60 min of embryo transfer, 76.4% (198/259) of the embryo flashes migrated towards the fundus, 12.4% (32/259) migrated towards the cervix and 11.2% (29/259) remained static. There was no significant association between the embryo position or movement and the pregnancy rate at 1 and 5 min. At 60 min, however, the pregnancy and implantation rates among subjects with embryo flashes located <15 mm from the fundus was significantly higher than those with embryo flashes located >15 mm from the fundus (46.5 and 32.8% versus 25.8 and 18.2%, respectively; P < 0.05). The pregnancy and implantation rates when the embryo flash was seen moving towards the cervix (25.0 and 15.0%) was significantly lower (P < 0.05 and P < 0.01, respectively) compared with those remaining static (55.2 and 37.7%) or moving towards the fundus (45.5 and 32.8%).

Limitations and reasons for caution: Although the air bubbles seen at the time of embryo transfer are thought to represent the position of the embryo, they are in fact a surrogate marker of the embryo itself, as this cannot be directly visualized by US.

Wider implications of the findings: Following embryo transfer the majority, but not all, embryo flashes undergo significant migration towards the fundus. The location of the embryo flash and the direction of its movement at 60 min, but not at 1 or 5 min after transfer, is associated with clinical outcome. These findings may challenge the traditional notion that the exact position of the embryo flash immediately following embryo transfer is related to clinical outcome.

Study funding/competing interests: The authors have no relevant funding or conflicts of interest to disclose.