['To assess the impact of cannulation of a resistant cervical os with the outer malleable sheath of a double-lumen, soft ET catheter on IVF-ET outcomes.', 'Retrospective cohort study.', 'University-based IVF center.', 'One hundred forty-two patients undergoing 142 ETs.', 'Trial ultrasound-guided ET at all transfers, leaving the malleable outer sheath in situ when the soft inner catheter could not negotiate the internal os.', 'Implantation rate and clinical pregnancy rate.', 'In 102 ETs (71.8%), the soft inner sheath easily negotiated the internal os (group 1). Forty ETs (28.2%) required cannulation of resistant internal ora with the outer sheath of the trial catheter (group 2). Implantation rates (35% vs. 32% in groups 1 and 2, respectively) and clinical pregnancy rates (50% vs. 45%) were not significantly different between groups. Blood was present on the transfer catheter after ET more frequently in group 2 than in group 1 (55% vs. 15%); however, neither the implantation rate nor the clinical pregnancy rate were affected by the presence of blood.'] Does cannulation of a resistant internal os with the malleable outer sheath of a coaxial soft embryo transfer catheter affect in vitro fertilization-embryo transfer outcome?