A woman's body undergoes enormous changes during pregnancy. The heart and kidneys must work harder due to increased blood volume: cardiac output increases 30-50% during pregnancy. Heart rate increases to 80-90 beats per minute. The enlarged uterus which reaches the lower edge of the rib cage by 36 weeks compresses the bladder and intestines making it necessary to urinate frequently and possibly causing constipation. The spine curves more to balance the weight of the growing uterus. The breasts enlarge and begin to produce colostrum in the final weeks of pregnancy.