Tocolytics as a class: nifedipine, indomethacin, contraindications and fetal monitoring

What tocolysis is meant to achieve

Tocolytics relax the uterus so that preterm labour slows for a limited time. Reference sources describe the realistic aim as delaying birth for around 48 hours so corticosteroids can be given to speed fetal lung maturation, and so the patient can be moved to a unit with neonatal intensive care. No tocolytic is clearly first line, so the choice is individualised.

The class includes calcium channel blockers such as nifedipine, prostaglandin synthesis inhibitors such as indomethacin, beta-adrenergic agonists such as terbutaline, and magnesium sulfate. Magnesium is now widely given for fetal neuroprotection before early preterm birth rather than as a labour-stopping drug. Long-term tocolysis to keep a pregnancy going for weeks is not recommended because it has not shown better outcomes. Antenatal corticosteroids are generally given between about 24 and 34 weeks, and magnesium is considered before 32 weeks to reduce the risk of cerebral palsy.

Situations where stopping labour is unsafe

Sometimes continuing the pregnancy is more dangerous than birth. Contraindications described in clinical reviews include fetal compromise, suspected chorioamnionitis, maternal haemodynamic instability and severe pre-eclampsia. Fever, uterine tenderness, foul discharge or maternal tachycardia should prompt a call before tocolytic doses continue.

Before the first dose, the nurse confirms gestational age, checks the fetal heart tracing and contraction pattern, takes baseline maternal vital signs and reviews conditions such as heart disease or poorly controlled diabetes that affect drug choice. Terbutaline carries a boxed warning: injectable terbutaline should not be used for prolonged tocolysis beyond 48 to 72 hours or for maintenance at home, because serious maternal reactions, including tachycardia, arrhythmias, pulmonary oedema, myocardial ischaemia, hyperglycaemia and hypokalaemia, have been reported.

Nifedipine: watching blood pressure and heart rate

Nifedipine blocks calcium entry into smooth muscle, relaxing the uterus and the blood vessels. The vascular effect causes the common adverse effects: dizziness, flushing, headache and hypotension, with a reflex rise in heart rate. The nurse measures blood pressure and pulse before doses and during the regimen, holds and reports symptomatic hypotension, and helps the patient change position slowly.

Reduced maternal blood pressure can reduce placental perfusion, so a falling maternal pressure and a changing fetal tracing should be reported together. Clinical reviews caution about cardiac effects when nifedipine and magnesium sulfate are combined. New breathlessness, chest pain or palpitations during treatment are reported promptly rather than attributed to anxiety.

Indomethacin: limits set by fetal physiology

Indomethacin reduces prostaglandin production, which lowers uterine contractility. The same mechanism affects the fetus. Fetal prostaglandins keep the ductus arteriosus open and support fetal kidney function, so prolonged use can narrow the ductus and reduce urine output, lowering amniotic fluid volume. Sources therefore limit tocolytic use to about 48 hours and avoid it after about 32 weeks.

The nurse confirms gestational age before giving indomethacin and reports if the course is extended beyond the planned window. The drug label also warns against non-steroidal anti-inflammatory use late in pregnancy because of ductus closure. Maternal concerns include gastrointestinal irritation and bleeding, so the nurse asks about ulcer history and gives doses with food if prescribed.

Testing the class in a hypothetical scenario

Picture an imagined patient at 30 weeks receiving nifedipine for preterm labour and betamethasone for fetal lungs. She now has a temperature of 38.4 C, a tender uterus and fetal tachycardia. The options are to give the next nifedipine dose, increase oral fluids, notify the provider about possible chorioamnionitis, or reassure her that steroids cause fever.

Notifying the provider is the best choice because intra-amniotic infection is a reason to stop tocolysis and may make birth the safer path. Giving the next dose continues a drug that may now be harmful, fluids do not address infection, and corticosteroids are not an expected cause of this picture. This is a study scenario only.

Sources and further reading

MSD Manual Professional: Preterm labor. Tocolytic classes, 48-hour aim for corticosteroids, individualised choice, prostaglandin inhibitor fetal risks and gestational limit, magnesium for neuroprotection.

American Family Physician: Preterm labor review. Nifedipine adverse effects, indomethacin limits, terbutaline cautions, contraindications to tocolysis and lack of benefit from long-term use.

DailyMed: Indocin (indomethacin) prescribing information. Avoidance late in pregnancy because of premature ductus arteriosus closure, and gastrointestinal bleeding warnings.

DailyMed: Terbutaline sulfate injection prescribing information. Boxed warning against prolonged or outpatient tocolysis and maternal cardiac and metabolic adverse reactions.