What is the appropriate treatment for a urinary tract infection in a woman in the third trimester of pregnancy?

Treatment of UTI in Third Trimester Pregnancy

For UTI in the third trimester, use cephalexin 500 mg four times daily for 7-14 days as first-line therapy, avoiding nitrofurantoin near term due to theoretical hemolytic risks in the newborn. 1

First-Line Antibiotic Options

Preferred Agents

• Cephalexin (500 mg four times daily for 7-14 days) is the preferred first-line alternative for third trimester UTIs, achieving adequate blood and urinary concentrations with excellent safety profiles 1
• Cefpodoxime or cefuroxime are also appropriate cephalosporin options for treating UTIs during pregnancy 1
• Fosfomycin (single 3g dose) can be considered for uncomplicated lower UTIs, though clinical data for third trimester use is more limited than for cephalosporins 1

Alternative Options

• Amoxicillin-clavulanate (20-40 mg/kg per day in 3 doses) is appropriate if the pathogen is susceptible 1
• Nitrofurantoin should be avoided in the third trimester due to theoretical risk of hemolytic anemia in the newborn, though it remains first-line for first and second trimesters 1

Critical Diagnostic Steps

Mandatory Testing

• Always obtain urine culture before initiating treatment to guide therapy and confirm the diagnosis 1
• Screening for pyuria alone has only 50% sensitivity for identifying bacteriuria in pregnant women and is inadequate 1
• Urine dipstick is unreliable in pregnancy with poor positive and negative predictive value 1

Follow-Up Testing

• Perform follow-up urine culture 1-2 weeks after completing treatment to confirm cure 1
• If symptoms persist or recur within 2 weeks, obtain repeat culture with antimicrobial susceptibility testing and retreat with a 7-day course of an alternative antibiotic 1

Treatment Duration and Approach

Standard Course

• The total course of therapy should be 7-14 days to ensure complete eradication of the infection 1
• For asymptomatic bacteriuria, 4-7 days is acceptable depending on the antimicrobial chosen 1

Asymptomatic Bacteriuria

• Pregnancy is the one clinical scenario where asymptomatic bacteriuria must always be treated due to 20-30 fold increased risk of pyelonephritis (from 20-35% untreated to 1-4% with treatment) 2, 1
• Treatment reduces premature delivery and low birth weight infants 2, 1

Special Considerations for Third Trimester

Group B Streptococcus (GBS)

• If GBS is identified in urine culture at any concentration, this indicates heavy genital tract colonization and requires both immediate treatment and intrapartum GBS prophylaxis during labor 1
• Women with GBS bacteriuria do not need vaginal-rectal screening at 35-37 weeks—they automatically qualify for intrapartum prophylaxis 1

Pyelonephritis Management

• For severe infections or pyelonephritis requiring hospitalization, initial parenteral therapy with ceftriaxone (1-2g daily) or cefepime (1-2g twice daily) is recommended 1
• Transition to oral cephalosporin therapy after clinical improvement for completion of 7-14 day course 1
• Agents that do not achieve therapeutic concentrations in the bloodstream, such as nitrofurantoin, should not be used for suspected pyelonephritis 1

Antibiotics to Avoid in Third Trimester

Contraindicated Agents

• Trimethoprim-sulfamethoxazole is contraindicated in the last trimester due to risk of kernicterus in the newborn 1
• Fluoroquinolones (ciprofloxacin) must be avoided throughout pregnancy due to potential adverse effects on fetal cartilage development 1
• Nitrofurantoin should be avoided near term (after 36 weeks) due to theoretical risk of hemolytic anemia in the newborn 1

Common Pitfalls to Avoid

• Do not rely on negative dipstick to rule out UTI—the presence of symptoms in pregnancy warrants culture and empiric treatment regardless of dipstick results 1
• Do not perform surveillance urine testing or treat asymptomatic bacteriuria repeatedly after the initial screen-and-treat approach, as this fosters antimicrobial resistance 1
• Do not delay treatment in pregnant women with symptomatic UTI, as this increases the risk of pyelonephritis and adverse pregnancy outcomes 1
• For recurrent UTIs, consider prophylactic antibiotics (cephalexin) for the remainder of pregnancy 1