Antenatal corticosteroids for preterm birth <34 weeks: Betamethasone 12 mg IM × 2 doses, 24 hours apart (or dexamethasone 6 mg IM q12h × 4 doses). Benefit window: 24–34 weeks (strongest evidence); consider at 22–23 weeks for periviable infants. Peak effect: 48 hours–7 days after first dose. Reduces RDS, IVH, NEC, and neonatal mortality by ~40%.
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