The normal hematologic adaptations to pregnancy protect the mother from one of the major effects of trauma, hemorrhage.
Pregnant women are physiologically prepared for hemorrhage.
Beginning early in pregnancy and culminating in the mid third trimester, plasma volume and red cell mass increase by 50 percent and 40 percent respectively. This extra blood fills the venous capacitance system, which is dilated. With blood loss, preload and cardiac output are maintained without significant changes in heart rate or blood pressure by using this extra reserve. Venous tone increases and recruits this reserve before vital signs are affected, at about 20 to 30 percent of blood volume, versus 10 percent in the nonpregnant state. Stable vital signs virtually preclude a serious major hemorrhage in the nonpregnant state, but are not so reassuring during pregnancy.