A 28-year-old primigravida at 32 weeks of gestation presents with vaginal bleeding and lower abdominal pain. On examination, the uterus is tender and contracting irregularly. Speculum examination shows blood in the vagina but cervix is closed. What is the most common cause of preterm labor in this clinical scenario?

Explanation

## Clinical Context The presentation of vaginal bleeding with uterine tenderness and contractions at 32 weeks strongly suggests **placental abruption**, which is a leading cause of preterm labor, particularly when accompanied by hemorrhage and maternal hemodynamic changes. ## Most Common Causes of Preterm Labor — Epidemiology | Cause | Frequency | Key Clinical Features | |-------|-----------|----------------------| | **Placental abruption** | 5–15% of preterm births | Vaginal bleeding, uterine tenderness, contractions, fetal distress | | **PPROM** | 25–30% of preterm births | Fluid leakage, absence of bleeding | | **Intrauterine infection** | 10–15% of preterm births | Maternal fever, elevated WBC, fetal tachycardia | | **Incompetent cervix** | 2–3% of preterm births | Painless cervical dilation, no bleeding | **Key Point:** While PPROM is the single most common *antecedent* to preterm labor overall, placental abruption is the most common *obstetric cause* presenting with acute hemorrhage and uterine contractions at mid-trimester. ## Pathophysiology of Abruption-Induced Preterm Labor 1. **Placental separation** → release of thromboplastin and tissue factor 2. **Activation of coagulation cascade** → thrombin generation 3. **Uterine irritation** → prostaglandin release (PGE₂, PGF₂α) 4. **Myometrial contraction** → preterm labor **Clinical Pearl:** The presence of *vaginal bleeding* + *uterine tenderness* + *contractions* is pathognomonic for abruption-triggered preterm labor. PPROM typically presents with fluid leakage without bleeding. **High-Yield:** Placental abruption accounts for ~5–15% of all preterm deliveries and is a medical emergency requiring immediate fetal monitoring and delivery planning. ## Why This Answer Fits the Vignette - Vaginal bleeding (classic sign of abruption) - Uterine tenderness (placental separation irritates myometrium) - Irregular contractions (prostaglandin-mediated) - Closed cervix (rules out PPROM as primary cause) - No mention of fever or infection (rules out chorioamnionitis) [cite:Williams Obstetrics 25e Ch 34]