Total blood volume increases during pregnancy because of a 50% increase in plasma volume and a lesser increase in red cell volume. This results in hemodilution and decreased hematocrit. With the increased blood volume, cardiac output increases early in pregnancy by 1 to 2 L/minute and is maintained until delivery. Despite the increased blood volume and cardiac output, systolic blood pressure remains essentially unchanged. This is probably due in part to the increased blood flow to the uterus (80% to the choriodecidua) and lowered peripheral vascular resistance.

During pregnancy, the mother’s serum creatinine generally decreases because of a 30% to 50% increase in both renal blood flow and glomerular filtration rate. Mean serum creatinine levels of 0.46 are common. Retention of sodium and water and renal wasting of glucose and amino acids are also noted during pregnancy. These changes are generally maintained up to term. By approximately 8 weeks postpartum, most physiologic changes of pregnancy can be expected to have returned to normal.

Pyeloureteral dilatation occurs commonly during pregnancy and is most prominent by weeks 22 to 24. The muscle-relaxing effects of increased progesterone during pregnancy are thought to play a
major role in addition to mechanical factors related to the fetus. A preponderance of right-sided involvement (˜75%) has been noted. Mechanical compression at the pelvic brim by the gravid uterus is the major cause. The left ureter appears to be somewhat protected from this compression by the sigmoid colon. Significant resolution of the condition can be expected within 24 to 48 hours after delivery. Urinary stasis is the most common adverse consequence of this partial obstruction; however, spontaneous rupture of the kidney has been known to occur. The bladder tends to be displaced anteriorly and superiorly by the growing uterus, producing more of an abdominal location than pelvic during pregnancy.