Since the baby is fed by your blood supply, and your enlarging reproductive organs require more blood flow, the amount of blood must also increase. Blood volume expands 25 to 40 percent.
To pump an increased amount of blood around the body, your heart must work slightly harder. The heart pumps more blood per beat and beats slightly faster. Heart murmurs attributable to the increased flow through the heart may develop.
The blood vessels are also affected by pregnancy. The enlarging uterus presses on veins in the pelvis, increasing the pressure in the veins that bring blood up from the legs. This increased pressure causes the leg veins to enlarge, producing varicosities (areas of enlargement). The pressure may also cause fluid to leak out of the veins and into the tissues, causing swelling of the feet and ankles. Late in pregnancy, the uterus can also compress a major vein in the abdomen, the vena cava, when you lie on your back; if this occurs, blood is prevented from returning to the heart, and a feeling of faintness may result.
The enlarged uterus also changes your digestive system during pregnancy. This affects your urination and digestion, and contributes to the well-known hunger cravings. Read about changes in the digestive system during pregnancy next.