## External Cephalic Version (ECV) and Tocolysis **Key Point:** Terbutaline is the preferred tocolytic agent for facilitating ECV in transverse lie and other malpresentations at ≥36 weeks. ### Mechanism of Action Terbutaline is a selective β~2~-adrenergic agonist that relaxes uterine smooth muscle, reducing uterine contractions and improving the ease of manual version. ### Why Terbutaline for ECV? | Agent | Onset | Duration | Advantage | Limitation | |-------|-------|----------|-----------|------------| | **Terbutaline** | 5–10 min | 4–6 hrs | Rapid onset, short half-life, minimal fetal effects | Maternal tachycardia, tremor | | Nifedipine | 10–20 min | 4–6 hrs | Longer acting, fewer maternal side effects | Slower onset; preferred for labor inhibition, not acute ECV | | Magnesium sulfate | 30–60 min | Variable | Neuroprotection if preterm | Slow onset, not ideal for ECV facilitation | | Labetalol | 5–10 min | 2–4 hrs | Antihypertensive | Non-selective β-blocker; **contraindicated** in asthma; less effective for uterine relaxation | **Clinical Pearl:** The goal of tocolysis during ECV is rapid uterine relaxation to reduce resistance to manual manipulation. Terbutaline's rapid onset (5–10 minutes) and short duration make it ideal for the acute procedure. **High-Yield:** ECV success rates improve from ~50% to ~60–70% with tocolytic use. Terbutaline IV or SC is standard in most obstetric units worldwide. **Warning:** Nifedipine, while excellent for maintenance tocolysis in preterm labor, has slower onset and is not preferred for acute ECV facilitation. Do not confuse the two clinical contexts. ### Contraindications to ECV - Previous cesarean delivery with classical incision - Placenta previa - Abnormal cardiotocography - Ruptured membranes - Maternal or fetal contraindications to vaginal delivery [cite:Williams Obstetrics 26e Ch 28]
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