Genitourinary Assessment and Renal Disease in Pregnancy
Katherine Ikard Stewart
Robert M. Ehsanipoor
KIDNEY AND URINARY TRACT DISORDERS
Renal Physiology in Pregnancy
The renal system undergoes many physiologic changes during a normal pregnancy. In addition, as the gravid uterus increases in size, it produces a mass effect on the renal system.
Structural changes: During pregnancy, the kidneys increase 1 to 1.5 cm in length and 30% in volume. The collecting system expands more than 80%, with greater dilation on the right side.
Mild right-sided physiologic hydronephrosis is seen as early as 6 weeks of gestation. Renal volume returns to normal within the first week postpartum, but hydronephrosis and hydroureter may not normalize until 3 to 4 months after delivery. Elective pyelography should, therefore, be deferred until at least 12 weeks postpartum.
These structural changes increase the risk of pyelonephritis in the setting of asymptomatic bacteriuria or urinary tract infections.
Renal filtration: Blood volume expansion during pregnancy increases renal plasma flow by 50% to 80%, which in turn results in an increased glomerular filtration rate (GFR). Increased GFR can be seen within 1 month after conception, peaking at 40% to 50% above prepregnancy levels by the end of the first trimester.
Elevated GFR increases creatinine clearance, so formulas for GFR based on age, height, and weight do not apply; creatinine clearance must be calculated with a 24-hour urine collection in pregnancy.
Increased GFR results in lower mean serum blood urea nitrogen (BUN) and serum creatinine during pregnancy (8.5 and 0.46 mg/dL, respectively). A serum creatinine which may be considered normal outside of pregnancy may suggest renal insufficiency in pregnancy.
Renal tubular function: Decreased tubular resorption in pregnancy increases urinary excretion of electrolytes, glucose, amino acids, and protein.
Increased calcium clearance is balanced by increased gastrointestinal (GI) tract absorption. Ionized calcium remains stable despite decreased total serum calcium because of the lower serum albumin concentration.
Physiologic hyponatremia occurs, with plasma sodium concentration falling by 5 mEq/L during pregnancy. Sodium levels return to baseline by 1 to 2 months postpartum.
Urinary excretion of glucose increases 10- to 100-fold, and glucosuria is observed routinely in normal pregnancy. Increased urinary glucose increases the risk of bacteriuria and urinary tract infections.
Renal resorption of bicarbonate decreases to compensate for the respiratory alkalosis of pregnancy, lowering serum bicarbonate by about 5 mEq/L in pregnancy.
Routine assessment of renal function: Proteinuria should be assessed at every prenatal visit. A urine dipstick value > 1 + should prompt further evaluation by clean-catch urine sample for culture and microscopy. If proteinuria persists despite negative culture, further evaluation is warranted and may include either a 24-hour urine protein collection or a random protein to creatinine ratio. A 24-hour total urine protein exceeding 150 mg is abnormal.
Patients with chronic hypertension, diabetes, preexisting renal disease, or other diseases may have abnormal levels of proteinuria prior to pregnancy and should undergo a baseline 24-hour urine protein collection early in pregnancy.
Serum creatinine persistently >0.9 mg/dL should prompt investigation for intrinsic renal disease. The presence of comorbidities should be assessed and further evaluation should be considered. Renal biopsy during pregnancy should be considered when the results will change management before delivery.
Urinary Tract Disorders in Pregnancy
Urinary Tract Infection
Urinary tract infections (UTIs) are common in pregnancy. Urinary stasis secondary to hydroureter and hydronephrosis, bladder trauma due to compression or edema, vesicoureteral reflux, and increased glucosuria may all contribute to the increased risk of infection. Women with two or more UTIs or a diagnosis of pyelonephritis during pregnancy should be considered for daily suppressive antibiotic therapy until delivery.
Asymptomatic bacteriuria (ASB) is the presence of bacteria within the urinary tract, excluding the distal urethra, without signs or symptoms of infection. ASB is associated with low-birth-weight infants and preterm delivery, and its treatment in pregnancy is indicated. The prevalence of ASB during pregnancy ranges from 2% to 7%. If left untreated, 20% to 30% of ASB in pregnant women progresses to pyelonephritis; treatment reduces this to 3%. Screening for bacteriuria with a urine culture is recommended at the first prenatal visit. Women with sickle cell trait have a twofold increased risk of ASB and can be screened every trimester.
A clean-catch urine culture with > 100,000 colonies/mL or catheterized urine culture with >100 colonies/mL warrants treatment.
Escherichia coli accounts for 75% to 90% of infections. Klebsiella, Proteus, Pseudomonas, Enterobacter, and coagulase-negative Staphylococcus are other common pathogens.
Initial therapy is usually empiric and may be altered based on urine culture sensitivities. Repeat urine culture is obtained 1 to 2 weeks after treatment and again each trimester. If bacteriuria persists after two or more treatment courses, suppressive therapy should be considered for the remainder of the pregnancy.
Acute cystitis occurs in approximately 1% to 3% of pregnant women. Symptoms include urinary frequency, urgency, dysuria, hematuria, and/or suprapubic discomfort. Empiric treatment regimens are the same as for ASB. If possible, a urine culture should be sent prior to initiating antibiotic therapy.
Urethritis is usually caused by Chlamydia trachomatis, and it should be suspected in patients with symptoms of acute cystitis and a negative urine culture. Mucopurulent cervicitis may also be present. The treatment of choice is azithromycin 1 g as a single oral dose for both the patient and her partner. A test of cure should be sent 3 to 4 weeks after treatment.
Pyelonephritis
Acute pyelonephritis occurs in approximately 1% to 2% of all pregnancies and is the leading cause of septic shock in pregnancy. Complications include preterm labor, preterm premature rupture of membranes (PPROM), bacteremia, sepsis, acute respiratory distress syndrome, and hemolytic anemia. Prompt diagnosis and treatment of pyelonephritis in pregnancy are crucial.
Symptoms include fever, chills, flank pain, nausea, and vomiting. Frequency, urgency, and dysuria are variably present.
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