Main findings:
This study showed that the inclination of uterine corpus might be an independent risk factor for the success of cleavage stage embryo transfer population. The more the uterine corpus tended to horizontal position, the lower the CPR and LBR of the cleavage-stage ET group. However, blastocyst transfer were not affected by the uterine corpus inclination. Moreover, there were no significant difference in pregnancy outcomes between cleavage stage embryo transfer and blastocyst transfer when the inclination of uterus was larger; when the uterus tends to the horizontal position (small uterus inclination ), the CPR and LBR of blastocyst transfer group were significantly higher than that of cleavage stage embryo transfer group. These findings suggest that active blastocyst transfer should be advised for the uterus tending toward the horizontal position, avoiding the influence of uterine position on pregnancy outcomes; for the patients with a uterus with greater inclination who have fewer embryos, blastocyst transfer should be avoided because of its non-superiority, to prevent the loss of precious embryos in the cultivation process 17.