After fertilization, the process of earlyhuman development includes a 5- to 7-day transit time for the embryo to move from the fallopian tube and be-come implanted in the endometrium, where placental development occurs. Af-ter implantation, the embryo becomes a fetus. Progesterone from the corpus lu-teum of the ovary prepares the estrogen-primed endometrium for implantation. Progesterone stimulates development of the maternal endometrial glandular and stromal cells needed for placental growth and development throughout pregnancy and quiets intrinsic uterine muscle con-tractility, which could mechanically dis-