Implantation, interception and contraception.

The factors involved in post-fertilization events leading to implantation in mammals are discussed with special reference to potential forms of interception. The stages of embryonic growth until implantation are considered initially. The growth and differentiation of the uterine endometrium is then described, followed by the events occurring during the apposition and invasion of the implanting embryo. Several potential approaches to new forms of interception are considered, and the advantages and disadvantages of each of them are evaluated. Among them, new vaccines against the zona pellucida, inactivation of the secretions of the blastocyst, hatching, the activity of pinopodes, and the endometrial proteins produced in the secretory phase seem to offer various and varied targets. Some existing methods of fertility regulation may act by affecting these stages of development, e.g. RU486 may interfere with pinopod function. Various physiological and embryonic consequences of interfering with these stages of pregnancy are discussed.