“The strongest ‘pound for pound’ muscle is the uterus: it weighs around 2 pounds but during childbirth can exert a downward force of 400 Newtons, which is one hundred times as strong as gravity and equivalent to the power in a fully extended modern longbow” –The Book Of General Ignorance by John Mitchinson
When not in use, a healthy uterus is a small organ, measuring about 3 inches (7.5 centimeters) long and 2 inches (5 cm) wide. During pregnancy, the uterus expands rapidly. By about 20 weeks into pregnancy, the growing uterus reaches all the way to the navel. By about 36 weeks, the uterus reaches the lower edge of the rib cage.
During pregnancy the uterus will increase it’s weight from about an ounce or two to a couple of pounds. It has the ability to stretch thanks to the round ligaments that anchor it to the pelvis and are able to stretch along with the growing uterus.
The uterus has the most oxytocin receptors in the body. When it comes time to give birth the uterus is flooded with hormones called prostaglandins which make the uterine cells more receptive to the hormone oxytocin.
The pregnant uterus processes one liter of blood per minute. During a uterine surge or “contraction” the uterus exerts as much as 43 pounds per square inch