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Kidney Changes During Pregnancy

The kidney undergoes tremendous anatomic and physiologic changes during pregnancy. There is a significant dilation of the upper urinary tract, primarily due to the mechanical obstruction of the ureters at the pelvic brim by the enlarged uterus. The dilatation is facilitated by the high levels of progesterone which is a smooth muscle relaxant.
Of particular importance is the fact that these anatomic changes may persist for up to 6 weeks postpartum and may be mistaken for other causes of obstructive uropathy.

In addition to the anatomic change, there is tremendous physiologic change as manifested by an increase in the glomerular filtration rate of between 40-50% of normal. This parallels the increases in blood volume and cardiac output . It begins as early as the 8th to 10th week of gestation and is evidenced by a decrease in the serum creatinine and BUN as well as an increase in the creatinine clearance. This is the only condition known to medicine that can increase the glomerular filtration rate that much. The decrease in serum creatinine and BUN is very important from a clinical standpoint. Pregnant patients who have a serum creatinine or BUN at the upper limit of normal as defined by the laboratory for non-pregnant individuals should be viewed with marked suspicion of renal impairment. The serum creatinine should normally be below 0.8 mg% and the BUN below 13 mg% in the pregnant patient. [ 1 ]

Plasma concentrations of electrolytes, glucose and urea, fall if more water than sodium, for example, is retained. Over the whole period of gestation there is retention of 7.5L of water and 900 mmol of sodium.

After the 12th week of gestation, progesterone can induce dilation and atony of the renal calyses and ureters. With advancing gestation, the enlarging uterus can compress the ureters as they cross the pelvic brim and cause further dilatation by obstructing flow. These changes may contribute to the frequency of urinary tract infections during pregnancy. [ 2 ]

The information in this page is presented in summarised form and has been taken from the following source(s):
1. Robert J. Blaskiewicz, M.D. Clinical Professor, Department of Obstetrics, Gynecology and Women's Health, Saint Louis University, U.S.A.:
2. World Anaesthesia & the World Federation of Societies of Anaesthesiologists :

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