Asymptomatic Bacteriuria in Pregnancy

Comments from Expert Advisory Committee

  • Asymptomatic bacteriuria is the presence of 1 or more species of bacteria growing in the urine at a quantitative count of >105 colony-forming units/mL, without signs or symptoms attributable to urinary tract infection.
  • Asymptomatic bacteriuria occurs in 2% to 7% of pregnant women.
  • Antibiotic treatment of asymptomatic bacteriuria reduces the risk of pyelonephritis, low birth weight and premature labour.
  • A urine culture is usually requested early in pregnancy (12-16 weeks).
  • The presence of asymptomatic bacteriuria should be confirmed by two consecutive urine cultures (preferably within two weeks).
  • Urine culture remains the gold standard in detection of asymptomatic bacteriuria.
  • Dipstick testing is not sufficient to screen for bacterial UTIs in pregnancy.
  • A positive urine culture for bacteriuria in the second sample of urine should be treated with an appropriate antibiotic for the bacteria isolated and the trimester of pregnancy.
  • Confirm clearance with urine culture after treatment.
  • Repeat urine cultures at subsequent antenatal visits may be required for prior recurrent UTI’s, diabetes mellitus, renal anomaly.
  • Trimethoprim should not be prescribed for pregnant women with established folate deficiency, low dietary folate intake, or women taking other folate antagonists.
  • Nitrofurantoin should be avoided after 36 weeks due to risk of neonatal haemolysis.


Drug Dose Duration +/- Notes
1st choice options- dependent on MSU cultures and susceptibility

Nitrofurantoin Immediate Release Capsules

50-100mg every 6 hours

7 days


Avoid after 36 weeks due to risk of neonatal haemolysis.
Immediate/ Prolonged Release should be stated on the prescription (1see note below on formulation difference)



Nitrofurantoin Prolonged Release Capsules 100mg every 12 hours 7 days
Amoxicillin* 500mg every 8 hours 7 days *Amoxicillin resistance is common – check susceptibility from MSU sample
2nd choice option- dependent on MSU cultures & susceptibility
Cefalexin 500mg every 8 hours 7 days  

If further information required on treatment options, please seek specialist/microbiologist advice.

*Seek specialist/microbiologist advice in case of severe penicillin allergy in pregnant patients if nitrofurantoin is not an option

1Two nitrofurantoin formulations are available: nitrofurantoin immediate release capsules (Macrodantin®) and nitrofurantoin prolonged release capsules (MacroBid®). For the treatment of infection the prolonged release capsules are dosed twice daily whilst the standard capsules are dosed four times daily. These products are not interchangeable.

Patient Information

Safe Prescribing (visit the safe prescribing page)

Reviewed May 2022