The physiologic reasons for this are: to minimize the effect of the erect and supine position on fetal and maternal circulation, to compensate for blood loss associated with parturition, and to meet the vascular needs of the enlarging uterus. Blood volumes are 45% above prepregnancy levels, and this same increase in blood volume is seen in patients with hydatidiform moles. The resting heart rate increases 10-15 beats/min during pregnancy. The heart is also displaced upward and to the left due to the ascension of the diaphragm during pregnancy (causing a slight left axis deviation on the ecg.) Arterial blood pressure and vascular resistance decrease overall during normal pregnancy. BPdecreases during the second trimester and slowly rises during the third trimester. Cardiac output at rest in the lateral recumbent position steadily increases during pregnancy.