Factors affecting embryo implantation after human in vitro fertilization: a hypothesis.

In the clinical practice of human in vitro fertilization, pregnancy is dependent on embryo implantation. Pregnancy is a function of the number of embryos transferred, with multiple embryos resulting in a higher likelihood of pregnancy. We formulated a mathematic model of embryo implantation. This model describes embryo implantation as dependent on three factors--transfer efficiency, embryo quality, and endometrial receptivity. Application of existing embryo implantation data to this model allows the calculation of the approximate value of each of these factors. On the basis of historic data, data obtained from our in vitro fertilization program, and these theoretic considerations, it is our hypothesis that (1) there is an inherent inefficiency associated with the mechanical transfer of embryos into the uterine cavity, which limits the maximal embryo implantation rate; (2) the quality of embryos produced by controlled ovarian hyperstimulation, follicle aspiration, and in vitro fertilization is very high and approaches that of embryos produced in natural cycles in vivo; and (3) endometrial receptivity is markedly diminished in stimulated cycles and is the current rate-limiting step of pregnancy success of in vitro fertilization.