During pregnancy, the woman’s heart must work harder because as the fetus grows, the heart must pump more blood to the uterus. By the end of pregnancy, the uterus is receiving one fifth of the woman’s prepregnancy blood supply. During pregnancy, the amount of blood pumped by the heart (cardiac output) increases by 30 to 50%. As cardiac output increases, the heart rate at rest speeds up from a normal prepregnancy rate of about 70 beats per minute to 80 or 90 beats per minute. During exercise, cardiac output and heart rate increase more when a woman is pregnant than when she is not. At about 30 weeks of pregnancy, cardiac output decreases slightly. Then during labor, it increases by an additional 30%. After delivery, cardiac output decreases rapidly at first, then more slowly. It returns to the prepregnancy level about 6 weeks after delivery.

Certain heart murmurs and irregularities in heart rhythm may appear because the heart is working harder. Sometimes a pregnant woman may feel these irregularities. Such changes are normal during pregnancy. However, other abnormal heart sounds and rhythms (for example, diastolic murmurs and a rapid, irregular heart rate), which occur more often in pregnant women, may require treatment.

Blood pressure usually decreases during the 2nd trimester but may return to a normal prepregnancy level in the 3rd trimester.

The volume of blood increases by almost 50% during pregnancy. The amount of fluid in the blood increases more than the number of red blood cells (which carry oxygen). Thus, even though there are more red blood cells, blood tests indicate mild anemia, which is normal. For reasons not clearly understood, the number of white blood cells (which fight infection) increases slightly during pregnancy and increases markedly during labor and the first few days after delivery.

Did You Know... The volume of a woman's blood increases by almost 50% during pregnancy.

The enlarging uterus interferes with the return of blood from the legs and the pelvic area to the heart. As a result, swelling (edema) is common, especially in the legs. Varicose veins commonly develop in the legs and in the area around the vaginal opening (vulva). They sometimes cause discomfort. Clothing that is loose around the waist and legs is more comfortable and does not restrict blood flow. Some measures not only ease the discomfort but may also reduce leg swelling and make varicose veins more likely to disappear after delivery: