## Tocolytic Agents in Preterm Labor **Key Point:** Nifedipine (a calcium channel blocker) is the first-line tocolytic agent in preterm labor between 24–34 weeks of gestation, particularly in the absence of contraindications. ### Rationale for Nifedipine Nifedipine is preferred because it: - Has the best safety profile with minimal maternal and fetal side effects - Does not require continuous fetal monitoring or maternal serum level checks - Can be administered orally (immediate-release formulation) - Reduces neonatal morbidity and mortality when used for fetal neuroprotection - Has no contraindication in most clinical scenarios ### Comparison of Tocolytic Agents | Agent | Mechanism | Onset | Route | Maternal Side Effects | Preferred Gestational Age | |-------|-----------|-------|-------|----------------------|-------------------------| | **Nifedipine** | Calcium channel blocker | 10–20 min | Oral/SL | Headache, flushing, hypotension (mild) | 24–34 weeks (first-line) | | Indomethacin | NSAID/COX inhibitor | 30 min | Oral/IV | GI upset, renal dysfunction | <32 weeks (short-term) | | Magnesium sulfate | NMDA antagonist | Immediate | IV | Flushing, nausea, weakness | Neuroprotection at <32 weeks | | Terbutaline | β₂-agonist | 5–10 min | SC/IV | Tachycardia, tremor, hyperglycemia | Rarely used now | **High-Yield:** Nifedipine is superior to β₂-agonists (terbutaline) and has a better side-effect profile than indomethacin for routine tocolysis. Magnesium sulfate is reserved primarily for neuroprotection, not as a first-line tocolytic. **Clinical Pearl:** In this patient at 32 weeks with no contraindications, immediate-release nifedipine 10–20 mg orally every 20–30 minutes (up to 3 doses) is the standard approach, followed by maintenance therapy if needed. **Warning:** Indomethacin should be avoided after 32 weeks due to risk of premature ductus arteriosus closure and oligohydramnios. Terbutaline carries maternal cardiovascular risks and is no longer recommended as first-line therapy. [cite:ACOG Practice Bulletin 127; Cunningham OB 26e Ch 42]
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