How does progesterone relax the uterus in pregnancy?

Most of us owe our existence to the calming influence of progesterone on our mother's uterine muscle (the myometrium). By the third trimester of pregnancy, the myometrium is akin to a sleeping giant. Once awakened, it becomes one of the strongest muscles in the human body to facilitate birth. How progesterone calms the myometrium for most of pregnancy is a major unanswered question in obstetrics.