Endocrine basis for the initiation, maintenance and termination of pregnancy in humans

Channels of hormonal communication between the embryo and the mother are established early in the conceptual process. The preimplantation embryo exerts local uterine and systemic effects on the maternal organism to provide a suitable environment for embryonic development, to enable the development of a receptive endometrium for implantation, and to maintain corpus luteum function. In primates, implantation can be induced with progesterone alone if the proliferative effects of estrogens on the endometrium have already been achieved; estrogens may have a priming effect for progesterone on the endometrium, but it is not obligatory for implantation. Progesterone secretion from the corpus luteum is essential for the maintenance of early pregnancy until the placenta takes over the major role of steroidogenesis. This luteo-placental shift is completed at about the 50th day of human gestation. Withdrawal of progesterone effects by the administration of the competitive progesterone antagonist, RU 486, within 24 days after conception will terminate 85% of human pregnancies. Other nonsteroidal functions of the corpus luteum have been recently suggested. Relaxin and prorenin are peptides secreted by corpus luteum of pregnancy but their role remains putative at the present time.