The length of mammalian pregnancy is tightly regulated to assure the delivery of a newborn mature enough to survive the extra-uterine environment. A successful pregnancy requires near complete relaxation of the uterus for more than ninety-five percent of gestation, overcoming the inherent tendency of the myometrium to contract with stretch. This active and highly regulated process is called myometrial quiescence. It requires not only the near absence of myometrial contractions, but also its refractoriness to contractile agents.