In a retrospective audit of 500 preterm deliveries at a tertiary center in India, infection-related causes were the most common identifiable etiology. Which of the following is the most common infectious cause of preterm labor?

Explanation

## Most Common Infectious Cause of Preterm Labor: Bacterial Vaginosis ### Epidemiology **Key Point:** Bacterial vaginosis (BV) is present in **10–30% of pregnant women** and is associated with a **2–3 fold increased risk** of preterm birth, making it the most common **preventable infectious cause** of preterm labor. ### Pathophysiology of BV-Induced Preterm Labor ```mermaid flowchart TD A[Bacterial vaginosis: Lactobacillus↓, Gardnerella/anaerobes↑]:::outcome A --> B[Ascending infection to upper genital tract]:::action B --> C[Chorioamnionitis & intrauterine inflammation]:::outcome C --> D[Prostaglandin release]:::action D --> E[Uterine contractions & preterm labor]:::outcome ``` ### Comparative Frequency of Infectious Causes | Infectious Cause | Frequency in PTL | Mechanism | Screening/Prevention | |------------------|------------------|-----------|---------------------| | **Bacterial vaginosis** | 20–40% of infection-related PTL | Ascending infection, prostaglandins | Clue cells on wet mount; treat if symptomatic | | **Urinary tract infection (UTI/pyelonephritis)** | 10–15% of infection-related PTL | Systemic inflammation, prostaglandins | Urine culture; antibiotics | | **Group B Streptococcus (GBS)** | 5–10% of infection-related PTL | Colonization (not invasive disease in most) | Vaginal/rectal swab at 35–37 weeks | | **Listeria monocytogenes** | < 1% of infection-related PTL | Rare; maternal bacteremia, placental invasion | Food-borne; very uncommon | **High-Yield:** BV is **asymptomatic in 50% of cases** but carries the same preterm risk as symptomatic BV. Screening and treatment of asymptomatic BV in high-risk women (prior preterm birth, multiple gestations) reduces PTL by ~30%. ### Diagnostic Criteria for BV (Amsel Criteria) At least 3 of 4: 1. Thin, homogeneous, gray-white vaginal discharge 2. Vaginal pH > 4.5 3. Positive "whiff test" (fishy odor after KOH addition) 4. Clue cells on wet mount (> 20% of epithelial cells) ### Treatment & Prevention - **Symptomatic BV:** Metronidazole 400 mg BD × 7 days (oral) OR clindamycin 300 mg BD × 7 days - **Asymptomatic BV in high-risk women:** Treat to reduce PTL risk - **Asymptomatic BV in low-risk women:** Treat only if symptomatic **Clinical Pearl:** GBS colonization alone does NOT cause preterm labor; it is treated only for **neonatal prevention** (intrapartum antibiotics at delivery). Invasive GBS disease (bacteremia, meningitis) is rare in pregnancy. **Mnemonic:** **ABCDE** of infection-related preterm labor: - **A**symptomaticBV (most common) - **B**acterial vaginosis (symptomatic) - **C**horioamnionitis (confirmed by amniocentesis) - **D**ental/periodontal disease (emerging evidence) - **E**nterovirus (rare) [cite:Williams Obstetrics 26e Ch 34; ACOG Practice Bulletin 190]