Mrs. So's edema is due to two of the normal conditions of pregnancy . First, a pregnant woman has more blood in her pregnant state than in her nonpregnant state. Second, the expanded uterus presses down on pelvic organs and blood vessels and impairs, if not obstructs, venous return. This effect is enhanced when the when the head of the baby enters the pelvic canal.
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It is quite normal for pregnant women to complain of symptoms of lightheadedness, faintness and dizziness, and to show signs of hemorrhoids, varicose veins, and pedal edema , These are are all consistent with impaired venous return produced by pressing of the pregnant uterus against the large pelvic blood vessels including the descending aorta, and the inferior vena cava. The pressure of the baby's head on pelvic blood vessels constricts the blood vessels and increases the intravascular hydrostatic pressure. As a consequence, bulk flow of fluid from blood vessels to interstitial space (ISS) increases, and reabsorption from ISS to blood vessels is retarded. The increased pressures of the expanded uterus pressure will also collapse lymphatic vessels as well as venules and small veins-- all contributing to accumulation of extracellular fluid in the ISS.