What is the best antibiotic for a urinary tract infection (UTI) in a 20-week pregnant woman?

Best Antibiotic for UTI at 20 Weeks Pregnant

For a symptomatic UTI at 20 weeks gestation, prescribe cephalexin 500 mg four times daily for 7 days as first-line therapy. 1

First-Line Treatment Options

Cephalosporins are the preferred choice for second trimester UTIs:

• Cephalexin 500 mg four times daily for 7-14 days is recommended by the American Academy of Pediatrics as first-line therapy 1
• Alternative cephalosporins include cefpodoxime or cefuroxime, all achieving adequate blood and urinary concentrations with excellent safety profiles in pregnancy 1
• The total course should be 7-14 days to ensure complete eradication 1

Nitrofurantoin is acceptable but has important limitations:

• Nitrofurantoin is recommended as first-line for uncomplicated lower UTI during first trimester 1
• However, nitrofurantoin should NOT be used if pyelonephritis is suspected, as it does not achieve therapeutic blood concentrations 1
• At 20 weeks, if there is any concern for upper tract involvement (fever, flank pain, systemic symptoms), avoid nitrofurantoin entirely 1

Fosfomycin is an alternative option:

• Single 3g dose can be considered for uncomplicated lower UTIs 1
• Clinical data for second/third trimester use is more limited compared to cephalosporins 1

Antibiotics to Avoid

Do not prescribe the following at any point in pregnancy:

• Trimethoprim-sulfamethoxazole: potential teratogenic effects, particularly in first trimester 1
• Fluoroquinolones (ciprofloxacin, levofloxacin): should be avoided throughout pregnancy due to adverse effects 1

Essential Management Steps

Obtain urine culture before initiating treatment:

• A urine culture should always be obtained to guide therapy 1
• This allows for adjustment if the organism is resistant to empiric therapy 1

Confirm cure with follow-up culture:

• Repeat urine culture 1-2 weeks after completing treatment to confirm eradication 1
• This is critical in pregnancy, as untreated or incompletely treated UTI can progress to pyelonephritis 1

Consider prophylaxis if recurrent:

• For recurrent UTIs during pregnancy, consider prophylactic cephalexin for the remainder of pregnancy 1

Clinical Algorithm

1. Obtain urine culture immediately 1
2. Assess for upper tract involvement (fever >38°C, flank pain, nausea/vomiting) 2
3. If uncomplicated lower UTI: Start cephalexin 500 mg QID for 7 days 1
4. If pyelonephritis suspected: Hospitalize and initiate IV therapy (ceftriaxone 1-2g daily or cefepime 1-2g BID) 2
5. Follow-up culture at 1-2 weeks post-treatment 1

Important Caveats

Amoxicillin-clavulanate requires caution:

• While amoxicillin-clavulanate (20-40 mg/kg/day in 3 doses) is appropriate if the pathogen is susceptible 1, one study reported an association with increased risk of necrotizing enterocolitis in neonates when used for premature rupture of membranes 3
• This concern is specific to prophylactic use in PROM, not treatment of active UTI, but warrants awareness 3

Asymptomatic bacteriuria must be treated in pregnancy:

• Unlike non-pregnant patients, asymptomatic bacteriuria in pregnancy should always be treated to prevent progression to pyelonephritis and adverse pregnancy outcomes 1
• Even a single positive culture warrants treatment 1