Which of the following is the most common identifiable cause of preterm labor in singleton pregnancies?

Explanation

## Etiology of Preterm Labor **Key Point:** Intrauterine infection (chorioamnionitis) is the single most common identifiable cause of preterm labor, accounting for 30–40% of cases with an identified etiology. ### Mechanism Bacterial colonization of the amniotic cavity triggers: 1. Inflammatory cytokine release (IL-6, IL-8, TNF-α) 2. Prostaglandin (PGE₂, PGF₂α) synthesis 3. Uterine contractions and cervical ripening 4. Rupture of membranes (in ~30% of cases) ### Common Organisms - *Ureaplasma urealyticum* - *Mycoplasma hominis* - *Gardnerella vaginalis* - *Group B Streptococcus (GBS)* - Gram-negative anaerobes ### Clinical Correlation **High-Yield:** Maternal fever, elevated WBC, tachycardia, and fetal tachycardia are clinical clues. Amniotic fluid culture or PCR is diagnostic but not routinely performed. ### Other Identifiable Causes (in order of frequency) | Cause | Frequency | Notes | |-------|-----------|-------| | Intrauterine infection | 30–40% | Most common | | Placental abruption | 5–10% | Vaginal bleeding, maternal pain | | Polyhydramnios | 2–5% | Excessive amniotic fluid | | Maternal hypertension | 2–3% | Preeclampsia/eclampsia | | Cervical insufficiency | 2–3% | Painless cervical dilation | **Clinical Pearl:** In ~50% of preterm labor cases, no identifiable cause is found (idiopathic preterm labor).