Physiologic Changes in Female’s Urinary Tract during Pregnancy

The rate of urine formation by a pregnant woman is usually slightly increased because of increased fluid intake and increased load or excretory products. But in addition, several special alterations of urinary function occur.

First, the renal tubules’ reabsorptive capacity for sodium, chloride, and water is increased as much as 50% as a consequence of increased production of steroid hormones by the placenta and adrenal cortex.

Second, the glomerular filtration rate increases as much as 50 per cent during pregnancy, which tends to increase the rate of water and electrolyte excretion in the urine.

Anatomy : Kidney

A remarkable number of changes are observed in the urinary system as a result of pregnancy (Table).

Kidney size increases slightly during pregnancy, the kidney was 1.5 cm longer during the early puerperium than when measured 6 months later using radiographs.

The glomerular filtration rate and renal plasma flow increase early in pregnancy, as much as 50 percent by the beginning of the second trimester, and the latter even greater. Both relaxin and neuronal nitric oxide synthase may be important for mediating the increased GFR and plasma flow during pregnancy. Elevated GFR persists until term, even though renal plasma flow decreases during late pregnancy.

Renal Changes in Normal PregnancyAlteration Clinical Relevance

Increased renal size Renal length approximately 1 cm greater on radiographs

Postpartum decreases in size should not be mistaken for parenchymal loss

Dilatation of pelves, calyces, and ureters

Resembles hydronephrosis on ultrasound or IVP (more marked on right)

Not to be mistaken for obstructive uropathy; retained urine leads to collection errors; upper urinary tract infections are more virulent; may be responsible for "distention syndrome" elective pyelography should be deferred to at least 12 weeks postpartum

Increased renal hemodynamics

GFR and renal plasma flow increase ~50%

Serum creatinine and urea nitrogen values decrease during normal gestation; > 0.8 mg/dL (> 72 umol/L) creatinine already suspect; protein, amino acid, and glucose excretion all increase

Changes in acid–base metabolism

Renal bicarbonate threshold decreases; progesterone stimulates respiratory center

Serum bicarbonate and Pco2 are 4–5 mEq/L and 10 mm Hg lower, respectively, in normal gestation; a Pco2 of 40 mm Hg already represents CO2 retention

Renal water handling

Osmoregulation altered: osmotic thresholds for AVP release and thirst decrease; hormonal disposal rates increase

Serum osmolality decreases 10 mOsm/L (serum Na 5 mEq/L) during normal gestation; increased metabolism of AVP may cause transient diabetes insipidus in pregnancy

*AVP =vasopressin; IVP = intravenous pyelography.

Kallikrein, a tissue protease synthesized in cells of the distal renal tubule, is found increased excretion at 18 and 34 weeks, which returned to nonpregnant levels at term.

As with blood pressure, maternal posture may have influence renal function. Late in pregnancy, urinary flow and sodium excretion less than half the excretion rate in the supine position compared with that in the lateral recumbent position.

Anatomy : Ureters

After the uterus rises completely out of the pelvis, it rests upon the ureters, laterally displacing and compressing them at the pelvic brim.

Hydroureter and hydronephrosis is from an effect of progesterone. The relatively abrupt onset of dilatation in women at midpregnancy, however, is more consistent with ureteral compression from an enlarging uterus rather than a hormonal effect.

Elongation accompanies distention of the ureter, which is frequently thrown into curves of varying size, the smaller of which may be sharply angulated.

Anatomy : Bladder

Only few significant anatomical changes in the bladder before 12 weeks.

From that time after that, the increased size of the uterus, together with the hyperemia that affects all pelvic organs, and the hyperplasia of the muscle and connective tissues, elevates the bladder trigone and causes thickening of its posterior, or intraureteric, margin. Continuation of this process produces marked deepening and widening of the trigone. The bladder mucosa undergoes no change other than an increase in the size and tortuosity of its blood vessels.

Toward the end of pregnancy, particularly in nulliparas in whom the presenting part often engages before labor, the entire base of the bladder is pushed forward and upward, converting the normal convex surface into a concavity. As a result, difficulties in diagnostic and therapeutic procedures are greatly increased. In addition, the pressure of the presenting part impairs the drainage of blood and lymph from the base of the bladder, often rendering the area edematous, easily traumatized, and probably more susceptible to infection.

Physiology : Loss of Nutrients

One unusual feature of the pregnancy-induced changes in renal excretion is the remarkably increased amounts of various nutrients in the urine. Amino acids and water-soluble vitamins are lost in the urine of pregnant women in much greater amounts than in nonpregnant women.

Physiology : Tests of Renal Function

The physiological changes in renal hemodynamics have several implications for the interpretation of tests of renal function.

Serum creatinine decrease from a mean of 0.7 mg/dL to 0.5 mg/dL, whereas values of 0.9 suggest underlying renal disease and should prompt further evaluation. Creatinine clearance in pregnancy should be 30% higher than the 100 to 115 mL/min normally measured in nonpregnant women. Creatinine clearance is a useful test to estimate renal function in pregnancy.

Serum urea nitrogen levels decrease from a mean 1.2 mg/dL to 0.9 mg/dL, whereas values of 1.4 mg/dL suggest underlying renal disease and should prompt further evaluation.

During the day, pregnant women tend to accumulate water in the form of dependent edema, and at night, while recumbent, they mobilize this fluid and excrete it via the kidneys. This reversal of the usual nonpregnant diurnal pattern of urinary flow causes nocturia, and the urine is more dilute than in the nonpregnant state.

Physiology : Urinalysis

Glucosuria during pregnancy is not necessarily abnormal. The appreciable increase in glomerular filtration, together with impaired tubular reabsorptive capacity for filtered glucose, accounts in most cases for glucosuria.

Proteinuria normally is not evident during pregnancy except occasionally in slight amounts during or soon after vigorous labor.

Hematuria most often suggests a diagnosis of urinary tract disease. Difficult labor and delivery can cause hematuria because of trauma to the lower urinary tract.

Urinary Tract Infection caused by Pregnancy

Urinary tract infections are more concerning in pregnancy due to the increased risk of kidney infections. During pregnancy, high progesterone levels decreased the muscle tone activity of the ureters and bladder, which leads to a greater likelihood of reflux, where urine flows back up the ureters and towards the kidneys.

Also, if bacteriuria is present in preganant women, they do have a 25-40% risk of a kidney infection. Thus if urine testing shows signs of an infection—even in the absence of symptoms—treatment is recommended. Cephalexin or nitrofurantoin are typically used because they are generally considered safe in pregnancy. A kidney infection during pregnancy may result in premature birth or pre-eclampsia .