How long is too long? The influence of the whole transfer procedure duration on pregnancy rates

Embryo transfer is a critical final step of IVF treatment. The definition of difficult transfer is subjective and not accurate. One of the characteristic that makes a transfer seem difficult is ''time consuming'', although there is no specific time cutoff identified. Limited studies examined the relationship between transfer time and pregnancy outcomes, with conflicting results. The duration of the whole procedure beginning with the passage of a trial catheter through the internal os, until the transfer is performed, was barely assessed. This study aimed to assess the effect of the duration of the entire transfer process on clinical pregnancy rates.

This was a single center retrospective cohort study, including fresh cycles with time recording between 2015 and 2023. The total duration was divided into four quartiles. All transfers were performed by staff physicians, under direct transabdominal ultrasound guidance. The procedure begins with the passage of a trial catheter through the internal os. The inner catheter is removed, and the loaded catheter is then passed through the outer sheath to the desired location within the endometrium. Data collected included basic demographic data, infertility diagnosis and ovarian reserve parameters. The primary outcome was the clinical pregnancy rate per transfer, defined as the presence of a gestational sac and a fetal heartbeat using ultrasonography one month after embryo transfer. Secondary outcomes included live birth rates and miscarriage rates. Statistical analysis included the Fisher-exact test and ANOVA. Multivariate logistic regression was performed to control for relevant confounders.

The study included 1079 fresh cycles. The total duration of the transfer ranges from 1-9 min. The study group was divided into four quartiles (minutes): Q1 (1-3.43), Q2 (3.44-4.88), Q3 (4.89-6.46), and Q4 (6.47-9). Female age was younger in Q3 and Q4 compared to Q1 (38.1 ± 5.2 (Q1) vs. 35.5 ± 5.7 (Q3) and 34.8 ± 6 (Q4), P = 0.001). There were no differences in BMI in the four groups (P = 0.31), no differences in cycle rank (P = 0.38), and no differences in gravidity and parity (P = 0.53 and P = 0.39, respectively). In univariate analysis, clinical pregnancy rates per transfer were higher in Q3 compared to Q1 (20.1% in Q1 compared to 31.1% in Q3; P = 0.003). The rate of live birth was higher in Q2 compared to Q1 (23.6% in Q2 vs. 7.7% in Q1, P = 0.003), and miscarriages in Q1 were higher compared to Q2 (40.6% vs. 16.8%, P = 0.002). In multivariate logistic regression, after controlling for relevant confounders, including embryo stage, grade and number of embryos transferred, and female age, longer transfer time was not negatively associated with pregnancy rates. With Q1 as reference, adjusted odds ratios (95% confidence interval) were 0.919 (0.538-1.571), 1.076 (0.631-1.834) and 0.627 (0.353-1.111) for transfer time groups Q2, Q3, and Q4, respectively.

A longer transfer time is not negatively associated with clinical pregnancy rates after controlling for relevant confounders. To support our findings, larger prospective studies should be conducted.