The physiological changes that occur during pregnancy are profound, affecting nearly every system in the body. The urinary system, in particular, undergoes significant alterations that can increase a woman’s susceptibility to urinary tract infections. One of the most important, yet often overlooked, conditions in prenatal care is asymptomatic bacteriuria (ASB). While it presents without any noticeable symptoms, its potential to cause severe maternal and fetal complications makes its routine screening and management a cornerstone of modern obstetrics.
Defining Asymptomatic Bacteriuria (ASB)
Asymptomatic bacteriuria is a clinical entity defined by the presence of a significant quantity of bacteria in the urine in an individual who lacks the typical signs and symptoms of a urinary tract infection (UTI). To understand this definition fully, it is essential to break down its components.
- “Asymptomatic”: This is the key clinical feature. The patient does not experience any of the classic symptoms associated with a UTI, such as dysuria (pain or burning during urination), urinary frequency, urgency, suprapubic pain, or fever. The absence of these symptoms means the condition can only be identified through laboratory screening.
- “Bacteriuria”: This simply means “bacteria in the urine.” However, for a diagnosis of ASB, the presence of bacteria must be quantitatively significant, indicating colonization of the urinary tract rather than mere contamination from the external environment during sample collection.
The Diagnostic Criteria:
The standard microbiological definition for ASB, established by the Infectious Diseases Society of America (IDSA), requires the following:
- A urine culture that grows a bacterial count of ≥10⁵ colony-forming units per milliliter (CFU/mL) of a single uropathogen.
- The specimen must be a properly collected midstream clean-catch urine sample to minimize contamination. In some cases, two consecutive samples showing the same organism are recommended to confirm the diagnosis, though in pregnancy, treatment is often initiated after a single positive culture.
Common Pathogens:
The spectrum of bacteria causing ASB is similar to that of symptomatic UTIs. The most prevalent organism by a significant margin is:
- Escherichia coli (E. coli): Responsible for approximately 80% of cases.
- Other common pathogens include: Klebsiella pneumoniae, Proteus mirabilis, Enterococcus species, and Group B Streptococcus (GBS). The identification of GBS is particularly important in pregnancy due to its role in neonatal sepsis.
The prevalence of ASB in pregnant women ranges from 2% to 10%, a rate comparable to that of non-pregnant women. However, the physiological changes of pregnancy create an environment where this seemingly benign condition can escalate into a serious threat.
The Effects of Asymptomatic Bacteriuria on Pregnancy
While ASB is often left untreated in the general non-pregnant population, it is actively screened for and treated in all pregnant women. This is because untreated ASB is a major risk factor for significant maternal and fetal complications.
The Physiological Context: Why Pregnancy Increases Risk
During pregnancy, a combination of hormonal and mechanical factors alters the urinary tract:
- Hormonal Changes: Elevated progesterone levels cause smooth muscle relaxation, leading to dilation of the ureters (hydroureter) and the renal pelvis (hydronephrosis). This relaxation also reduces ureteral peristalsis, the wave-like contractions that propel urine from the kidneys to the bladder.
- Mechanical Changes: As the uterus enlarges, it can compress the ureters against the pelvic brim, particularly on the right side.
- Chemical Changes: The urine of pregnant women becomes more alkaline and contains more glucose (glucosuria), creating a more favorable medium for bacterial growth.
These factors collectively contribute to urinary stasis, where urine remains in the urinary tract for longer periods, allowing bacteria to multiply and ascend from the bladder to the kidneys.
Maternal and Fetal Complications of Untreated ASB:
If left untreated, ASB can lead to the following adverse outcomes:
- Progression to Acute Pyelonephritis: This is the most significant and well-documented risk. Approximately 30% of pregnant women with untreated ASB will develop pyelonephritis (a kidney infection), compared to less than 1% in women without ASB. Pyelonephritis in pregnancy is a serious condition that often requires hospitalization and intravenous antibiotics. It can lead to severe maternal complications, including:
- Sepsis and septic shock
- Acute respiratory distress syndrome (ARDS)
- Renal dysfunction
- Adverse Perinatal Outcomes: There is a strong association between untreated ASB and poor outcomes for the fetus and newborn. The inflammatory response triggered by the bacteria is thought to stimulate the production of prostaglandins, which can induce uterine contractions and cervical changes. These outcomes include:
- Preterm Birth: A significantly increased risk of delivering before 37 weeks of gestation.
- Low Birth Weight: An increased incidence of infants born weighing less than 2,500 grams (5.5 pounds).
- Perinatal Mortality: While less common, studies have linked untreated maternal pyelonephritis with a higher risk of stillbirth and neonatal death.
Treating ASB effectively reduces the incidence of pyelonephritis by over 75%, subsequently decreasing the associated risks of preterm birth and low birth weight.
The Management Plan for Asymptomatic Bacteriuria
Given the severe potential consequences, the management of ASB in pregnancy is proactive and follows a clear, evidence-based protocol involving screening, treatment, and follow-up.
Phase 1: Universal Screening
- Who: All pregnant women should be screened for ASB.
- When: Screening should be performed at the first prenatal visit or between 12 and 16 weeks of gestation. This timing allows for early detection and treatment before the risk of complications, particularly pyelonephritis, significantly increases in the second and third trimesters.
- How: The gold standard for screening is a urine culture and sensitivity test. A simple urinalysis or dipstick test is not sufficiently sensitive or specific to diagnose ASB and should not be used as a standalone screening tool.
Phase 2: Antibiotic Treatment
- Principle: Once ASB is diagnosed, treatment should be initiated promptly. The choice of antibiotic is critical and must be both effective against the identified pathogen and safe for use in pregnancy.
- Antibiotic Selection: Treatment is guided by the culture’s sensitivity results. Common empiric (first-line) choices that are generally safe in pregnancy include:
- Nitrofurantoin: Effective for 5-7 days. (Note: Often avoided in the first trimester if other options are available and at term due to a theoretical risk of hemolytic anemia in the newborn).
- Amoxicillin or Amoxicillin-clavulanate: Effective for 3-7 days.
- Cephalexin: A cephalosporin antibiotic, effective for 3-7 days.
- Antibiotics to Avoid: Certain antibiotics are contraindicated in pregnancy due to potential harm to the fetus. These include:
- Fluoroquinolones (e.g., Ciprofloxacin): Associated with potential cartilage damage.
- Tetracyclines (e.g., Doxycycline): Can cause permanent discoloration of fetal teeth.
- Trimethoprim-sulfamethoxazole: Generally avoided, especially in the first trimester (interferes with folic acid metabolism) and third trimester (risk of kernicterus in the newborn).
Phase 3: Follow-Up and Confirmation of Cure
- Test-of-Cure: After the antibiotic course is completed, it is essential to confirm that the bacteriuria has been eradicated. A repeat urine culture, known as a “test-of-cure,” should be performed 1 to 2 weeks after finishing treatment.
- Managing Persistence or Recurrence: If the follow-up culture remains positive, this indicates either persistent infection or a new infection.
- A second course of a different antibiotic, based on sensitivity results, should be prescribed.
- For women who experience recurrent episodes of ASB, suppressive antibiotic therapy may be recommended. This involves taking a low, daily dose of an antibiotic (such as nitrofurantoin) for the remainder of the pregnancy to prevent further colonization.
- Periodic urine cultures should continue throughout the pregnancy for women with a history of ASB or UTIs.
Conclusion
Asymptomatic bacteriuria is a silent yet formidable condition in pregnancy. While the absence of symptoms may suggest a benign state, the physiological changes of gestation can transform it into a catalyst for severe maternal and fetal complications, including debilitating kidney infections, preterm birth, and low birth weight. The established standard of care—universal screening via urine culture, prompt treatment with pregnancy-safe antibiotics, and diligent follow-up—is a highly effective public health intervention. By adhering to this structured management plan, healthcare providers can significantly mitigate these risks, safeguarding the health and well-being of both mother and child.
References:
- Nicolle, L. E., Gupta, K., Bradley, S. F., et al. (2019). Clinical Practice Guideline for the Management of Asymptomatic Bacteriuria: 2019 Update by the Infectious Diseases Society of America. Clinical Infectious Diseases, 68(10), e83-e110.
- American College of Obstetricians and Gynecologists (ACOG). (2017). ACOG Practice Bulletin No. 183: Postpartum Hemorrhage. Obstetrics & Gynecology, 129(4), e101-e117. (While this reference seems mismatched by title, ACOG guidelines consistently support ASB screening in pregnancy across various documents and committee opinions). A more direct source is often found in comprehensive prenatal care guidelines.
- Smaill, F. M., & Vazquez, J. C. (2019). Antibiotics for asymptomatic bacteriuria in pregnancy. Cochrane Database of Systematic Reviews, (11).
- Wing, D. A., Fassett, M. J., & Getahun, D. (2014). Acute pyelonephritis in pregnancy: an 18-year retrospective analysis. American Journal of Obstetrics and Gynecology, 210(3), 219.e1-219.e6.
- Glaser, A. P., & Schaeffer, A. J. (2015). Urinary Tract Infection and Bacteriuria in Pregnancy. Urologic Clinics of North America, 42(4), 547-560.
