This chapter discusses the roles played by the ovary in the initiation, maintenance, and termination of pregnancy in viviparous vertebrates. Having formed and discharged the egg, the ovary reverts to an intermediary role so that a consideration of its function falls naturally into two parts: (1) the factors that influence or control ovarian activity and (2) the effects of that activity upon other organs. The delay in the recurrence of estrus is associated with the persistence of the corpus luteum beyond its duration in the normal cycle. The relation between this organ and the maintenance of pregnancy was first appreciated in the early years of the century. The chapter describes the ovarian changes in pregnancy and the formation of the corpus luteum in eutherian mammals. The outstanding ovarian change involved in pregnancy is the increased functional survival of the corpus luteum and, as a consequence, the predominance of progesterone in the ovarian steroid output. Progesterone is described as the hormone of pregnancy for without a source of progesterone, no pregnancy can be maintained.
