Introduction
In vitro fertilization and embryo transfer (IVF-ET) is a globally utilized form of assisted reproductive technology, and ET is the final and most essential step thereof 1. During the ET process, the fertilized embryos are cultured in vitro and placed directly inside the uterine lumen via an ET catheter2. Previous studies investigating the main factors affecting the implantation rate reported endometrial receptivity and embryo quality as risk factors 3. However, ET, as the last and probably most underestimated step, has not been developed as much as the other aspects of the IVF process. Studies have shown that transferred embryos do not always implant in situ, despite the transfer process being performed under ultrasound guidance 4, 5. The catheter loading, catheter position, injection speed, uterine peristalsis and patients’ posture during ET may affect the migration of embryos 4, 6, 7. Poindexter et al. reported that approximately 15% of the transferred embryos were expelled from the uterine cavity and were found in the cervical os, fallopian tubes, or the ET catheter 8. Baba demonstrated that approximately 80% of embryos implant in areas to which they initially are transferred and approximately 20% implant in other areas9. The migration of the transferred embryos might make detrimental effect on a successful pregnancy, and also predisposes the patient to ectopic pregnancy 10.
A laboratory simulation of ET into a uterine model demonstrated that the dispersion of the transferred matter depend on the position of sagittal cross-section of the uterine cavity with respect to the horizon6. However, the in vitro model can not represent the real situation in vivo, and the cleavage stage embryos and blastocysts might be affected differently by the inclination of uterus due to their different migration duration before implantation. Therefore, it remains unclear whether the inclination of uterine corpus affects the pregnancy outcomes of IVF-ET.
This study investigated whether the inclination of the uterine corpus affect the pregnancy outcomes in IVF-ET, and whether different uterine inclination affect the pregnancy outcomes of embryos at different developmental stages.