women s health issues
Physical Changes During Pregnancy
Urinary tract
Like the heart, the kidneys work harder throughout pregnancy. They filter the increasing volume of blood. The volume of blood filtered by the kidneys reaches a maximum between 16 and 24 weeks and remains at the maximum until just before the baby is due. Then, pressure from the enlarging uterus may slightly decrease the blood supply to the kidneys. Activity of the kidneys normally increases when a person lies down and decreases when a person stands. This difference is amplified during pregnancy—one reason a pregnant woman needs to urinate frequently while trying to sleep. Late in pregnancy, lying on the side, particularly the left side, increases kidney activity more than lying on the back. Lying on the left side relieves the pressure that the enlarged uterus puts on the main vein that carries blood from the legs. As a result, blood flow improves and kidney activity increases. The uterus presses on the bladder, reducing its size so that it fills with urine more quickly than usual. This pressure also makes a pregnant woman need to urinate more often and more urgently. 