Effects of Asymptomatic Bacteriuria in Pregnancy
Untreated asymptomatic bacteriuria in pregnancy dramatically increases the risk of pyelonephritis by 20-30 fold (from 1-4% with treatment to 20-35% without treatment) and is associated with preterm delivery and low birth weight infants. 1
Maternal Effects
Pyelonephritis Risk
- Women with asymptomatic bacteriuria in early pregnancy have a 20-30 fold increased risk of developing pyelonephritis during pregnancy compared to women without bacteriuria 1
- The absolute risk of pyelonephritis in untreated asymptomatic bacteriuria ranges from 20-35%, which drops to 1-4% with antimicrobial treatment 1, 2
- Implementation of screening and treatment programs has reduced pyelonephritis rates in all pregnant women from 1.8-2.1% down to 0.5-0.6% 1
Other Maternal Complications
- Asymptomatic bacteriuria may be associated with hypertensive disease, anemia, and postpartum urinary tract infections, though the evidence for these associations is less robust 3
Fetal and Neonatal Effects
Preterm Birth
- Untreated asymptomatic bacteriuria increases the risk of preterm delivery 1, 4
- Treatment with antimicrobials reduces preterm birth risk from approximately 53 per 1000 to 14 per 1000 (risk difference of -39 per 1000) 5
- The Cochrane review found antibiotic treatment may reduce preterm birth (RR 0.34,95% CI 0.13 to 0.88), though the certainty of evidence is low 6
Low Birth Weight
- Asymptomatic bacteriuria is associated with increased rates of low birth weight infants 1, 4
- Antimicrobial therapy reduces the risk of very low birth weight from approximately 137 per 1000 to 88 per 1000 (risk difference of -49 per 1000) 5
- Meta-analyses support that treatment decreases the frequency of low birth weight infants, though methodological limitations exist in older studies 1, 7
Fetal Mortality
- Higher fetal mortality rates have been reported in association with untreated asymptomatic bacteriuria 3
Evidence Quality and Clinical Context
The evidence supporting treatment of asymptomatic bacteriuria in pregnancy is remarkably consistent across multiple studies and time periods, despite most foundational studies being conducted early in the antimicrobial era 1. The consistency and robustness of observations from multiple prospective trials resulted in screening for and treating asymptomatic bacteriuria during pregnancy becoming a standard of care in developed countries 1, 2.
Pregnancy is the one clinical scenario where asymptomatic bacteriuria must always be treated due to the significant risk of progression to pyelonephritis and adverse pregnancy outcomes 8, 5. This distinguishes pregnant women from other populations where asymptomatic bacteriuria is generally not treated 2.
Common Pitfalls
- Do not rely on urinalysis or dipstick alone—urine culture is mandatory for diagnosis, as screening for pyuria has only 50% sensitivity for identifying bacteriuria in pregnant women 8, 5
- Do not delay treatment while waiting for culture results in symptomatic cases, but always obtain culture before initiating therapy 8
- Asymptomatic bacteriuria occurs in 2-7% of pregnant women and requires active screening at 12-16 weeks gestation or at the first prenatal visit 2, 5