What is a tocolytic?

What is a Tocolytic?

A tocolytic is a medication designed to inhibit uterine contractions by suppressing myometrial smooth muscle activity, used primarily to delay preterm delivery for 48-72 hours to allow time for corticosteroid administration and maternal transfer to appropriate facilities. 1, 2, 3

Primary Purpose and Clinical Context

• Tocolytics do not prevent preterm birth or improve long-term neonatal outcomes—their sole purpose is to gain 48-72 hours for critical interventions. 1, 2

• The time gained allows for:

  • Administration of antenatal corticosteroids for fetal lung maturation 2, 4
  • Magnesium sulfate for fetal neuroprotection (if <32 weeks) 2, 4
  • Transfer to a tertiary care facility with appropriate NICU capabilities 1, 4

Recommended Gestational Age Window

• Tocolytics should be considered between 24-34 weeks gestation when there is preterm labor with cervical change and no contraindications. 1, 2, 4

• At advanced cervical dilatation (≥7 cm), tocolysis is futile and preparation for imminent delivery is recommended instead. 1

First-Line Tocolytic Agents

• Nifedipine (calcium channel blocker) is the preferred first-line agent, particularly extended-release formulations for ease of once-daily administration. 4, 5

• Indomethacin (NSAID) is an acceptable alternative but should be used with caution considering gestational age due to potential fetal effects (risk of premature ductus arteriosus closure and oligohydramnios). 4, 3

• Both agents may effectively delay delivery for 48-72 hours in women with preterm labor and intact membranes after 26 weeks. 2, 4

Critical Safety Considerations

• Never combine nifedipine with magnesium sulfate—this combination can induce uncontrolled hypotension and fetal compromise. 4

• Short-acting nifedipine should be avoided except in low-resource settings, as it can cause dangerous hypotension, particularly when combined with magnesium sulfate. 4

• Common side effects of nifedipine include headaches, tachycardia, and edema. 4

Other Tocolytic Agents

• Magnesium sulfate is used as a tocolytic, though its primary benefit is for fetal neuroprotection rather than tocolysis, reducing cerebral palsy incidence when given before anticipated early preterm birth (<32 weeks). 2, 4, 6

• Beta-adrenergic agonists (ritodrine, terbutaline) can delay delivery but are associated with significant maternal side effects including tachycardia, pulmonary edema, and metabolic disturbances. 3, 7

• Progesterone has been studied for primary, adjunctive, and maintenance tocolysis, but there is currently insufficient evidence to recommend it for these indications. 8

Important Clinical Caveats

• Antibiotics are not recommended for preterm labor with intact membranes—they provide no benefit and may cause harm (increased risk of necrotizing enterocolitis with amoxicillin-clavulanic acid). 4

• For preterm prelabor rupture of membranes (PPROM) after 24 weeks, antibiotics are strongly recommended using a 7-day regimen (IV ampicillin and erythromycin for 48 hours, followed by oral formulations for 5 days). 4

• Combination tocolytic therapy lacks evidence for superiority over single agents and may increase adverse effects. 7

• Tocolysis is contraindicated when delivery would be beneficial for maternal or fetal indications (e.g., chorioamnionitis, severe preeclampsia, fetal distress, significant hemorrhage). 9

Mechanism of Action

• Tocolytics work through various mechanisms to reduce myometrial excitability: calcium channel blockers prevent calcium influx, NSAIDs inhibit prostaglandin synthesis, beta-agonists increase intracellular cAMP, and magnesium competes with calcium at the cellular level. 10, 3