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.