Screening for Asymptomatic Bacteriuria in Pregnancy
Screen all pregnant women for asymptomatic bacteriuria with urine culture at 12-16 weeks' gestation (or at the first prenatal visit if later), and perform this screening only once during early pregnancy. 1
Recommended Screening Timeline
The correct answer from your options is 12 weeks (specifically 12-16 weeks' gestation). This is a Grade A recommendation from the U.S. Preventive Services Task Force, representing the highest level of evidence. 1
Key Timing Considerations:
Single screening at 12-16 weeks' gestation is the standard of care, as recommended by both the USPSTF and the American College of Obstetricians and Gynecologists 1, 2, 3
If the patient presents later than 16 weeks, screen at the first prenatal visit regardless of gestational age 1, 2
Do not routinely screen at 20,26, or 32 weeks unless there are specific clinical indications or the patient was not screened earlier 1
Rationale for Early Screening
Why 12-16 Weeks Specifically:
Prevents pyelonephritis: Untreated asymptomatic bacteriuria leads to pyelonephritis in 20-35% of pregnant women, representing a 20-30 fold increased risk compared to non-pregnant women 3, 4
Reduces adverse pregnancy outcomes: Treatment of bacteriuria detected early in pregnancy significantly reduces preeclampsia, preterm labor, preterm premature rupture of membranes, intrauterine growth restriction, and low birth weight 5, 6
Timing matters for effectiveness: Women screened and treated early (by 20 weeks) have significantly better outcomes than those screened later (32-34 weeks), even when both groups receive treatment 5
Screening Method
Urine culture is mandatory - this is the only acceptable screening test 1, 2
Do not use urinalysis alone: Standard urinalysis (including pyuria testing) has poor sensitivity (~50%) and cannot replace urine culture 2, 3
Positive result definition: ≥10^5 CFU/mL of a single uropathogen in a clean-catch midstream specimen 1, 3
Follow-Up After Initial Screening
If initial culture is negative: Women have only a 1-2% risk of developing pyelonephritis later in pregnancy, so routine repeat screening is not necessary 2
If treated for positive culture: Perform periodic screening for recurrent bacteriuria throughout the remainder of pregnancy 3
Recurrence is common: Approximately 21.7% of treated women will have recurrence requiring repeat treatment 6
Common Pitfalls to Avoid
Screening too late: Waiting until 20+ weeks significantly reduces the protective benefit against pregnancy complications, even if treatment is provided 5
Using urinalysis instead of culture: This misses approximately 50% of cases and leads to both false positives and false negatives 2
Screening multiple times unnecessarily: A single negative culture at 12-16 weeks is sufficient for low-risk women 1, 2
Not screening at all: The prevalence of asymptomatic bacteriuria in pregnancy is 2-10%, and screening with treatment reduces pyelonephritis from 1.8% to 0.6% 6, 4