Tocolytics

Tocolytics suppress uterine contractions to delay preterm delivery (typically between 24–34 weeks gestation), primarily to buy 48 hours for corticosteroids to mature fetal lungs. They do not prevent preterm birth — they stall it. Terbutaline is a beta-2 adrenergic agonist. By stimulating beta-2 receptors on uterine smooth muscle, it causes relaxation and stops contractions. The tradeoff: beta-2 receptors also exist in the heart and lungs, so terbutaline causes maternal tachycardia, tremors, hyperglycemia, and hypokalemia. Maternal heart rate above 120 bpm or chest pain warrants holding the dose. The FDA issued a black box warning against IV terbutaline use beyond 48–72 hours due to risk of pulmonary edema and maternal death. Nifedipine, a calcium channel blocker, relaxes uterine smooth muscle by blocking calcium influx needed for contraction. Its primary adverse effect is hypotension and reflex tachycardia. You monitor blood pressure before each dose — hold if systolic falls below 90 mmHg. Nifedipine should not be given concurrently with magnesium sulfate because both lower blood pressure and suppress calcium-dependent processes, creating dangerous hypotension and neuromuscular blockade risk. Neither drug is used if the cervix is dilated beyond 4–6 cm, membranes have ruptured with infection, or there is fetal distress requiring delivery.