Interpretation
The natural position of the uterus depends on the uterine ligament and the pelvic floor muscles and fascia. It affects the incidence of some gynecological diseases and postoperative complications. A study in 1987 reported that retroversion of uterus was positively correlated with the occurrence of endometriosis 18. Ante-version and retroversion of uterus are more likely to suffer from chronic pelvic pain, dyspareunia and dysmenorrhea 19, and a higher angle of flexion was associated with increased menstrual pain20. Cesarean section scar diverticulum is more prevalent in patients with a retro-flexed uterus than in those with an ante-flexed uterus 21. Retroversion of uterus was significantly associated with occurrence of intraoperative complications such as uterine perforation during curettage 22, 23. The pathogenesis of all these diseases is related to the physical aspect and anatomical characteristics of the uterus. IVF is a non-physiological process and ET is an human operated invasive procedure, we believe that the uterine corpus inclination might be an independent risk factor for implantation of the cleavage-stage embryos.
Our results demonstrated that the more the uterine corpus tends to horizontal position, the lower the CPR and LBR of the cleavage-stage ET group. This result is in consist with the conclusion of laboratory model in vitro which demonstrated that the transfer liquid is more likely to be pulled backward toward the cervix in the uterus with horizontal position, which may make adverse effect on pregnancy rate6. However, the CPR and LBR of the blastocyst transfer cohort were not affected by the inclination of uterus in this study. We speculate that it may be related to the different migration duration of the two kinds of embryos before implantation. The cleavage-stage embryos need to migrate for 3-4 days before implantation 6, whereas the blastocysts implant immediately after deposition24. We hypothesized that the longer the embryo migration duration, the more likely the embryo migration trajectory was affected by the reduced inclination of the uterine corpus; moreover, the grafts might flow downward under the influence of gravity in a less inclined uterus, so that the embryos might leave the optimal implantation position and affect pregnancy outcomes. In addition, this study demonstrated that the pregnancy outcomes were not statistically difference between anteverted uteri and retroverted uteri (OR 1.030, 95% CI 0.677-1.568), which was consistent with the prospective study conducted by Egbase et al. 25.
Our results contradict the previous understanding that the pregnancy rate of blastocyst transfer is always higher than that of cleavage-stage ET 26. We found no significant difference in the CPR, SAR and LBR between the cleavage-stage ET and blastocyst transfer cohorts when the distance was under 2.146 cm (which represent a more tiled uterus). In contrast, when the distance exceeded 2.146 cm (which represent a uterus more tend to horizontal position), the CPR and LBR of the blastocyst transfer cohort was significantly higher than that of the cleavage-stage ET cohort. This indicates that active blastocyst transfer should be advised for patients with a uterus tending toward the horizontal position, avoiding the influence of uterine position on pregnancy outcomes. For the patients with a uterus with greater inclination and fewer embryos, blastocyst transfer should be avoided because of its non-superiority, to prevent the loss of precious embryos in the cultivation process 17..
This study has made a potential contribution to the effect of uterine corpus inclination on embryo implantation in IVF-ET. A multicenter randomized controlled trial with a more accurate measurements are needed to further confirm this conclusion, and that is what we are doing right now.