Effect of embryo transfer catheter incubation strategy on pregnancy and live birth outcomes: a retrospective comparative cohort study

Technical Summary: Effect of Embryo Transfer Catheter Incubation Strategy on Pregnancy and Live Birth Outcomes

Problem Statement
While assisted reproductive technology (ART) has advanced significantly in embryo culture and selection, the final embryo transfer (ET) step remains a critical variable influencing implantation and live birth rates. Although embryos are cultured in tightly controlled environments, the procedural interval between embryo loading and intrauterine deposition can expose the embryo-containing catheter to transient cooling. Experimental evidence suggests that temperature fluctuations can disturb temperature-sensitive cytoskeletal structures in gametes and that catheter contents can cool rapidly during routine handling. Despite existing guidelines emphasizing soft catheter use and ultrasound guidance, there is no consensus on the optimal pre-incubation strategy for the specific components of soft ET systems (the inner catheter carrying the embryo versus the outer guiding sheath). This study addresses whether the specific incubation strategy of catheter components is associated with clinical pregnancy and live birth outcomes.

Methodology
This was a retrospective comparative cohort study conducted across tertiary ART centers in Goa, India, analyzing cycles performed between January 2024 and July 2025.

• Participants: 240 women aged 21–40 years undergoing fresh or frozen-thawed ET were included. Cycles were excluded based on uterine anomalies, thin endometrium (<6 mm), use of donor gametes, or incomplete follow-up.
• Intervention Groups: Cycles were categorized based on the documented catheter handling protocol:
  • Group A (Inner only): The sterile inner catheter was incubated in a CO2 incubator at 37°C for ≥15 minutes; the outer sheath remained at room temperature.
  • Group B (Outer only): The sterile outer sheath was incubated at 37°C for ≥15 minutes; the inner catheter remained at room temperature until loading.
  • Group C (Combined): Both the inner catheter and outer sheath were incubated at 37°C for ≥15 minutes.
• Procedure: All transfers utilized the same soft catheter model, performed by experienced clinicians under ultrasound guidance. Embryo loading was standardized to ≤60 seconds.
• Outcomes: Primary outcomes were clinical pregnancy (fetal cardiac activity) and live birth (delivery >24 weeks). Secondary outcomes included biochemical pregnancy, ongoing pregnancy, implantation rate, and miscarriage.
• Analysis: Statistical comparisons used ANOVA/Kruskal-Wallis and Chi-square/Fisher exact tests. Multivariable logistic regression adjusted for maternal age, embryo stage, transfer type, endometrial thickness, number of embryos, and embryo quality.

Key Results

• Baseline Characteristics: Demographic, clinical, and embryological variables were comparable across all three groups (P > 0.05).
• Temperature Profiles: Quality-control measurements confirmed significant thermal differences. Group B (Outer only) exhibited the lowest inner catheter/loading-column temperature at loading (34.1°C), while Group C (Combined) maintained the most stable profile (36.5°C). Group A (Inner only) showed intermediate stability (36.2°C).
• Clinical Outcomes:
  • Clinical Pregnancy: Group C had the highest rate (58.8%), followed by Group A (52.5%), and Group B (35.0%) (P = 0.008).
  • Live Birth: Group C achieved 50.0%, Group A 43.8%, and Group B 27.5% (P = 0.011).
  • Comparative Analysis: Compared to Group B, Group A showed an absolute live birth difference of 16.3 percentage points, and Group C showed a 22.5 percentage point difference.
• Multivariable Analysis: After adjusting for confounders, both Group A (Adjusted OR 1.97, 95% CI 1.02–3.82) and Group C (Adjusted OR 2.56, 95% CI 1.31–4.98) remained significantly associated with higher odds of live birth compared to Group B.

Key Contributions

• Differentiation of Component Incubation: The study provides empirical data distinguishing the impact of warming the inner catheter versus the outer sheath. It demonstrates that incubating the outer sheath alone (Group B) yields significantly lower success rates than strategies involving the inner catheter.
• Thermal Correlation: The findings link the physical temperature profile of the catheter loading column directly to clinical outcomes, supporting the hypothesis that minimizing the temperature gradient at the embryo-medium interface is clinically relevant.
• Evidence for Protocol Refinement: The data suggests that the "inner catheter" is the critical thermal component, challenging protocols that prioritize warming the outer sheath without ensuring the inner catheter is pre-warmed.

Significance and Claims
The authors claim that in this multi-center retrospective cohort, incubating the embryo-carrying inner catheter—either alone or in combination with the outer sheath—is associated with higher clinical pregnancy and live birth rates compared to incubating the outer sheath alone.

The study concludes that the thermal stability of the inner catheter is a modifiable procedural factor that may influence ART success. However, the authors maintain a modest stance regarding routine adoption. They emphasize that while the findings are biologically plausible and statistically significant in this cohort, the retrospective nature of the study and the use of simulated temperature measurements (rather than direct embryo-level monitoring) necessitate caution. The paper explicitly calls for prospective, multi-center studies with standardized temperature monitoring to confirm these findings before such protocols are incorporated into routine standard operating procedures.