Urinary Tract Infection (UTI) During Pregnancy

1. Why Pregnancy Increases UTI Risk

• Ureteral hypotonia: Progesterone reduces ureteral muscle tone and peristalsis, slowing urine flow
• Physiologic hydronephrosis: The enlarging uterus mechanically obstructs the ureters, causing dilation of the upper ureters and renal pelvices (right-sided in ~75%, left-sided in ~33% of women by mid-pregnancy)
• Bladder changes: Decreased tone, increased capacity, and incomplete emptying predispose to vesicoureteric reflux
• Urine chemistry: Rising urinary pH (increased bicarbonate excretion) and glycosuria both favor bacterial growth
• Increased estrogen excretion: Accelerates growth of uropathogenic E. coli strains
• Result: Static columns of urine in ureters facilitate upward migration of bacteria from the bladder to the kidneys
• Creasy & Resnik's Maternal-Fetal Medicine, Principles and Practice

2. Classification of UTI in Pregnancy

Key risk: If ASB is not identified and treated, up to 40% of infected pregnant women will develop acute pyelonephritis.

3. Causative Organisms

• Comprehensive Clinical Nephrology, 7th Edition; Tintinalli's Emergency Medicine

4. Diagnosis

• Screening: All pregnant women should be screened at least once for ASB, ideally by urine culture (dipstick alone is unreliable due to frequent contamination from vaginal secretions)
• Dipstick: Positive leukocyte esterase or nitrites should be treated; negative result should still prompt culture if clinical suspicion exists
• Culture: Required for definitive diagnosis; a positive leukocyte esterase or nitrite on dipstick warrants treatment empirically while awaiting culture results
• Reagent strips have limited sensitivity for ASB in pregnancy

5. Clinical Presentations

Asymptomatic Bacteriuria

• No symptoms by definition
• Still requires treatment because of the risk of progression to pyelonephritis

Acute Cystitis

• Dysuria, frequency, urgency, suprapubic pain
• No systemic features

Acute Pyelonephritis

• Most commonly presents between 20-28 weeks' gestation
• Fever, rigors, flank/loin pain (usually right-sided), nausea, vomiting
• Not always preceded by lower urinary tract symptoms
• Can present as acute abdominal pain - a diagnostic pitfall
• May trigger preterm labor (via proinflammatory cytokines from bacterial endotoxins)
• Bacteremia is a common complication; sepsis can develop with AKI, DIC, and ARDS

6. Complications

7. Treatment

Safe Antibiotics in Pregnancy

Antibiotics to AVOID in Pregnancy

• Fluoroquinolones: Cartilage toxicity concerns in the fetus - avoid unless no alternative
• Tetracyclines: Fetal bone/tooth effects
• Trimethoprim alone in 1st trimester: Folate antagonist effects

Pyelonephritis Treatment

• Most experts recommend initial hospitalization for 12-24 hours for observation, even for apparently mild disease
• In the second half of pregnancy: almost all patients should be hospitalized for parenteral antibiotics given the risk of preterm labor
• IV crystalloid fluid resuscitation to maintain urine output ≥30-50 mL/hour
• Monitor renal function, urine output, and respiratory status (pulse oximetry)
• After clinical improvement: complete 10-14 days of oral antibiotics
• Suppressive therapy after pyelonephritis: nitrofurantoin 100 mg at bedtime for the remainder of pregnancy (reduces recurrence from 60% to <10%)
• Creasy & Resnik's Maternal-Fetal Medicine; Tintinalli's Emergency Medicine

8. Recurrent UTI and Follow-Up

• Recurrent infections are driven by: persistent bacteriuria, glycosuria, and mechanical ureteric compression in the third trimester
• Options after treating pyelonephritis:
  1. Daily suppressive therapy (more cost-effective)
  2. Urine cultures every 2 weeks for the duration of pregnancy
• Reflux nephropathy increases the risk of sudden hypertension and worsening renal function

Recent Evidence Update

1. UTI and preterm birth - A 2024 meta-analysis (PMID 39258657) confirms UTI is a significant risk factor for preterm birth
2. Bacteriuria and adverse outcomes - A 2024 meta-analysis (PMID 38073146) found associations between bacteriuria and preterm birth and low birth weight, though the magnitude varies
3. Consensus on lower UTI management - Delphi consensus 2024 (PMID 38213058) addresses open questions including management in pregnancy

Note: The 2024 meta-analysis on bacteriuria and adverse outcomes raises some uncertainty about whether routine treatment of ASB still confers the same benefit as historically assumed - the evidence quality from older trials is low, and a recent RCT found no association between ASB and adverse outcomes. This is an evolving area, but current guidelines still recommend treatment of ASB in pregnancy.

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