The muscular outer layer of the uterus, which surrounds the inner layer known as the endometrium. The myometrium is the culprit for menstrual cramps, as it contracts to expel the engorged bloody layer which develops in fertile women in preparation for pregnancy. If fertilization did in fact occur, the myometrium will remain quiescent until it's time for birth, at which time it will kick into action and work hard to push the baby out through the vagina. Ouch.