## Discriminating Feature: Cervical Dilation and Uterine Contractions ### Key Distinction Between Preterm Labor and Placenta Previa **Key Point:** Preterm labor is defined by the triad of uterine contractions (≥2 contractions in 10 minutes), cervical change (dilation ≥2 cm or effacement ≥80%), and gestational age 20–37 weeks. Placenta previa presents with painless vaginal bleeding without cervical dilation or contractions. ### Comparison Table | Feature | Preterm Labor | Placenta Previa | | --- | --- | --- | | **Vaginal bleeding** | Absent or minimal | Painless, often heavy | | **Uterine contractions** | Present (≥2 in 10 min) | Absent | | **Cervical dilation** | Present (≥2 cm) | Absent | | **Abdominal pain** | Present (labor-type) | Absent | | **Speculum findings** | Cervical dilation visible | Blood in vault, cervix closed | | **Ultrasound** | Normal placental location | Placenta covers internal os | ### Clinical Pearl **Clinical Pearl:** The cardinal sign of preterm labor is **progressive cervical change** (dilation and/or effacement) accompanied by regular uterine contractions. Vaginal bleeding alone, even with abdominal discomfort, does not constitute preterm labor if the cervix remains closed and contractions are absent. ### High-Yield Mnemonic **Mnemonic: LABOR** — **L**abor contractions, **A**bdomen pain (labor-type), **B**irth-ready cervix (dilation/effacement), **O**pen cervix, **R**egular pattern (≥2 contractions in 10 min). Placenta previa has none of these. ### Why This Matters In this case, the patient has vaginal bleeding and pain, but the speculum shows a closed cervix with no mention of dilation. The absence of cervical dilation and documented uterine contractions rules out preterm labor and points to placenta previa as the diagnosis.
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