Strengths and limitations
MINOR, CAVITY
We conducted a large RCT with sufficient power to identify a significant difference between CPR with 4D UGET vs CTT, including an unselected population of 320 women recruited and randomised on the day of their ET.A statistical difference was noted between the ages of the women in the per protocol analysis, but the marginal difference in mean ages (33 and 34 years), is unlikely to be clinically relevant. The ITT analysis revealed no significant difference in ages.A curved catheter was required to perform 4D UGET, as the curve in the catheter helps to stabilise the catheter upon removal of the speculum and insertion of the TV ultrasound probe. Without the curve and bung the speculum could not be removed allowing the TV ultrasound probe to be re-introduced. Thus, the catheters utilised in our control and study groups were different. The differing catheters may pose an inherent bias in our data; however, both of these catheters are soft catheters and previous studies have confirmed non-inferiority between different soft cathetersA 1–2 s time lag between the 4D ultrasound images was noted during the trial and had to be accounted for when performing the transfer. We overcame this issue with slow and steady catheter insertion, and good communication between the performing practitioner and the embryologist. Future advances in ultrasound technology are expected to further refine this minor issue.4D ultrasonography is an advanced technique and is not a skill available in every assisted reproductive technology (ART) unit. Additional training is required to develop the technique for performing this type of ultrasonography and it is currently untested against 2D UGET. Therefore, considering the additional cost and resource implications, we are cautious in recommending its routine use.All of the 4D UGET and 95% of the CTT were performed by one experienced operator and although this reduced operator bias, we are not able to determine the effect or experience of different practitioners on the outcomes. We documented the duration of the procedure from the start until deposition of the embryo, however, the length of time from when the embryo leaves culture until it is deposited in the uterine cavity may be of more relevance as a surrogate of stress to the embryo.