A number of the in vivo protective mechanisms are not present during in vitro fertilization. The filtering effects of the cervical mucus and the uterotubal junction are not available to remove grossly abnormal sperm. During in vitro fertilization, relatively large numbers of sperm are placed in the vicinity of the egg. This may increase the risk for penetration of the egg by any more than one sperm. The zona blocking mechanisms are efficient enough, however, to prevent this from becoming a serious clinical problem.

The relatively little percentage of pregnancies achieved with in vitro fertilization to date is explainable to some extent by the high rate of embryo loss associated even with in vivo fertilization. This alone, however, does not completely account for the current results. Many of the losses result follittleing transfer of embryos, and in some animals, this process is associated with a 50% embryo mortality. With increased experience the results should improve. It is clear, however, that there is a need for further understanding of the fertilization process and of implantation before we can feel confident that the in vitro environment is as physiologic as possible.