Slower embryo transfers linked to lower IVF pregnancy rates
image: The impact of transfer time on clinical pregnancy outcomes. The transfer time was converted into a categorical variable by tertiles. After adjusting for age, BMI, blood contamination, and the number of retrieved oocytes, the clinical pregnancy rate in group T3 was significantly lower than that in group T1.
Credit: Chu, Da-Peng; Fu, Yao; Bai, Xue-Yan.
A new study suggests that how long an embryo spends outside the incubator during IVF transfer significantly affects the chances of a successful pregnancy, with transfers taking longer than 34 seconds associated with notably lower success rates.
Published in Reproductive and Developmental Medicine, the study by researchers from Beijing Chao-Yang Hospital analyzed 486 frozen embryo transfer cycles at a reproductive medicine centre in Beijing. All transfers involved two high-quality frozen embryos and were performed by a single clinician, ensuring that differences in outcomes could be more confidently attributed to transfer time rather than other variables.
During IVF, embryos are loaded into a thin catheter and transferred into the uterus. "Transfer time" refers to how long this process takes from the moment embryos are loaded into the catheter to the moment they are released into the uterine cavity. Crucially, no temperature control is applied during this stage and embryos are briefly exposed to ambient room temperature rather than the stable 37°C environment of the laboratory incubator.
Patients were divided into three groups based on how long their transfer took: 15–27 seconds, 28–33 seconds, and 34–81 seconds. Clinical pregnancy rates were 55.6%, 51.2%, and 41.1%, respectively. Implantation rates followed a similar pattern, falling from around 36% in the two faster groups to 25.5% in the slowest. The researchers identified 34 seconds as a critical threshold: beyond this point, embryo viability appears to be compromised, likely due to the effects of temperature drop on early embryonic development.
The authors recommend that clinics work to minimise transfer times where possible, for example, by reducing the physical distance between the laboratory workstation and the procedure room, and by using transfer simulators to train practitioners.