Asymptomatic bacteriuria in pregnancy - Screening and treatment

Publication: 19/03/2021  
Next review: 19/03/2024  
Clinical Guideline
ID: 6903 
Approved By: IAPG 
Copyright© Leeds Teaching Hospitals NHS Trust 2021  


This Clinical Guideline is intended for use by healthcare professionals within Leeds unless otherwise stated.
For healthcare professionals in other trusts, please ensure that you consult relevant local and national guidance.


NICE recommends that all pregnant women with asymptomatic bacteriuria (ASB) should receive a seven day course of antibiotics to prevent persistent bacteriuria, preterm birth, and low birth weight.


All pregnant women should be offered screening at the time of booking.

All urine samples for Microbiology must be freshly collected and sent in a sterile red top (boric acid) container, as per normal MSU collection instructions.

Requesting on ICE: select one of the following from the antenatal urine culture (reason for sending) menu:

All patients (at booking)

MSU sent as ‘ASB screen 1’

Patients with bacteriuria present on the 1st screen

MSU sent as ‘ASB screen 2’

Women who have Group B Strep isolated in their urine (including on a single screen)

Initial course of oral antibiotics directed by the sensitivity results and then ensure intra-partum antibiotic prophylaxis1 is offered to all these women

ASB screen result with ‘mixed growth’, or with different organisms

Send repeat MSU of that screen (i.e. if Screen 1 is mixed enteric flora, send a repeat sample as ‘ASB screen 1’)

Women seen in the Preterm prevention clinic

MSU sent as ‘preterm prevention clinic’ sample

All women treated for asymptomatic bacteriuria in pregnancy

Send a ‘test of cure’ MSU on completion of treatment (within seven days of finishing antibiotics)

If symptomatic please send MSU stating symptomatic and refer to the UTI in pregnancy guideline.

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Empirical treatment should not be given for asymptomatic bacteriuria in pregnancy.  If the patient becomes symptomatic please refer to the relevant UTI in pregnancy guideline.

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When ASB has been confirmed with 2 screening MSUs (same organism and susceptibilities) please use the table below to select the appropriate agent as per the susceptibility results.

Order of preference

Antibacterial and dose


Duration: 7 days

1st line

Nitrofurantoin PO 50mg 6-hourly

Avoid after 36 weeks gestation3
Avoid in patients with a history of abnormal renal function2

2nd line

Amoxicillin electronic Medicines Compendium information on Amoxicillin PO 500mg 8-hourly

Avoid in penicillin allergy

3rd line

Trimethoprim PO 200mg 12-hourly

Avoid before 12 weeks gestation

4th line

Cefalexin electronic Medicines Compendium information on Cefalexin PO 500mg 8-hourly

Avoid in penicillin allergy (unless known to tolerate cephalosporins)

Please contact Microbiology to discuss further options if required.

Please discuss with a senior obstetrician if the ‘test of cure’ sample returns the same result as the ASB screens.


  • Please see intrapartum prophylaxis guidance here
  • Nitrofurantion should not be used if CrCl <45 mL/min. Do not rely on eGFR as this may over-estimate renal function
  • The BNF states to “avoid at term - may produce neonatal haemolysis”. Author consensus was to use 36 weeks. 


Record: 6903
Clinical condition:

Asymptomatic bacteriuria in pregnancy

Target patient group:
Target professional group(s): Pharmacists
Secondary Care Doctors
Adapted from:

Evidence base

Approved By


Document history

LHP version 1.2

Related information

Not supplied

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