During pregnancy, a woman's body must ramp up many of its systems to support the growing fetus inside of it. The cardiovascular system has to pump 30 to 50 percent more blood, sending much of the extra blood supply directly to the uterus. The increase in blood and circulation also means more work for the kidneys and lungs.
During the birthing process, the cervix expands to about 10 centimeters to allow the baby's head to pass through the birth canal. The event can be a strenuous one for both mother and baby, and both bodies adjust to accommodate it. Blood output from the mother's heart jumps, and hormone levels shift. Babies are forced to adapt quickly to their new environment:, and they start to breathe with their lungs (rather than getting oxygenated blood from the mother) and regulate their own body temperature.