What Are Tocolytics?

In situations when a pregnant person begins showing signs of preterm labor—which is any time before 37 weeks of completed pregnancy—their doctor may recommend attempting to delay the process, using a category of drugs called tocolytics. The aim of postponing labor is to give the fetus more time to fully develop in the womb, and, as a result, avoid the potential complications associated with preterm birth, including breathing difficulties and impaired vision.

This article discusses the purpose, types, risk factors, and side effects of tocolytics.

Purpose

While a typical human pregnancy lasts about 40 weeks—from the start of a person's last menstrual period to childbirth—the gestation period can be cut short for a variety of reasons. Any time labor starts prior to hitting the 37-week mark, it's considered preterm (or premature) labor, which can result in a preterm birth.

But all preterm births don't carry equal risk. For example, a baby born after 36 weeks' gestation may have a lower-than-average birthweight, but also benefits from having (close to) fully developed organs—with the exception of their lungs, which may still need more time. But if a baby is born after only 29 weeks' gestation, for example, not only will their lungs likely be immature, but their brain may still be developing as well.

Though outcomes can vary significantly among neonates, some of the more common categories of health challenges associated with preterm birth include:

• Breathing problems
• Vision problems
• Hearing problems
• Feeding difficulties
• Cerebral palsy
• Developmental delays
• Blood glucose regulation

So when a pregnant person shows signs of preterm labor, their healthcare team will likely attempt to safely postpone the actual labor and delivery, in order to give the fetus more time to finish developing. This is done through a procedure called tocolysis, in which medications from a specific category of drugs—known as tocolytics (toh-coh-LIT-iks)—are used to slow or stop contractions of uterine smooth muscle. Tocolytics may help delay labor by two to seven days.

In addition to tocolytics, healthcare providers may also prescribe corticosteroids— like betamethasone or dexamethasone—to help increase the speed of fetal development before childbirth takes place. While corticosteroids are not considered tocolytics (given that they have a different function), the two types of medications are often prescribed together, especially if the preterm labor begins between 24 and 34 weeks of gestation.

Finally, it's important to keep in mind that tocolytics alone have not been shown to directly improve neonatal outcomes: Their function is to help delay the labor process itself. This is why they're often used in conjunction with corticosteroids.

Types

Several different classes of drugs are used for tocolysis, including:

• Betamimetics (such as terbutaline)
• Magnesium sulfate
• Prostaglandin inhibitors (like indomethacin, ketorolac)
• Calcium channel blockers (such as nifedipine)
• Nitrates (like nitroglycerine)
• Oxytocin receptor blockers (such as atosiban)

There is not currently a single medication that has emerged as the safest and most effective tocolytic, so medical professionals have to weigh the risks and benefits of the different options for each person. This includes:

• Considering how a particular drug works
• Its potential side effects
• Any contraindications the person may have
• How complex it is to administer

Risk Factors

There are a number of contraindications—or conditions that make a treatment unsafe for a person—for the use of tocolytics. These include:

• A gestational age above 34 weeks
• Intrauterine fetal demise
• Lethal fetal anomaly
• Nonreassuring fetal status
• Severe preeclampsia or eclampsia
• Maternal bleeding with hemodynamic instability
• Chorioamnionitis
• Preterm premature rupture of membranes (except when there are no signs of maternal infection and there is a need for transportation, steroid administration, or both)
• Specific contraindications to tocolytic agents
• Advanced cervical dilation (over 5 centimeters)
• Maternal heart disease
• Hyperthyroidism
• Uncontrolled diabetes
• Mild abruptio placentae
• Stable placenta previa
• Intrauterine growth restriction

Side Effects

Because several different classes and types of drugs fall under the category of tocolytics, there is not a single, standard list of side effects. Here are some of the side effects of tocolytics, broken down by drug class:

Betamimetics

• Drugs in the class: Ritodrine, terbutaline, and salbutamol
• Major side effects: Cardiac arrhythmias (tachycardia), hypotension, hyperglycemia, pulmonary edema

Calcium channel blockers

• Drugs in the class: Nifedipine, nicardipine
• Major side effects: Maternal hypotension, dizziness

Magnesium sulfate

• Drug in the class: Magnesium sulfate
• Major side effects: Flushing, respiratory suppression, cardiac arrest

Oxytocin receptor blockers

• Drugs in the class: Atosiban (not available in the United States)
• Major side effects: Gastrointestinal upset

Prostaglandin inhibitors

• Drugs in the class: Indomethacin, sulindac, celecoxib
• Major side effects: Maternal gastrointestinal disturbance, oligohydramnios (too little amniotic fluid surrounding the fetus during pregnancy), premature constriction of the ductus

Nitrates and others

• Drugs in the class: Nitroglycerin, nitric oxide
• Major side effects: Headache, flushing, maternal hypotension, tachycardia

Summary

Tocolytics are a category of drugs used to delay the labor process. These may be used in situations when a pregnant person begins showing signs of preterm labor—which is any time before 37 weeks of completed pregnancy.

A Word From Verywell

In addition to tocolytics and corticosteroids, there are increasingly advanced therapeutic and incubation methods in the event of a preterm birth. Your healthcare providers will be able to walk you through different options and discuss which might work best for you.