Several medications are used to inhibit preterm contractions, with nifedipine (a calcium channel blocker) and indomethacin (an anti-inflammatory) being the most widely favored first-line options. These drugs, collectively called tocolytics, don’t stop preterm birth permanently. Their primary goal is to delay delivery by 48 hours, buying enough time to administer corticosteroids that dramatically improve the baby’s chances of surviving and avoiding serious complications.
That 48-hour window matters more than it might sound. Babies whose mothers receive corticosteroids before a preterm delivery have roughly 34% lower rates of respiratory distress syndrome, 46% lower rates of brain bleeding, and 31% lower rates of death compared to babies who don’t get that head start.
Nifedipine: The Most Common Choice
Nifedipine is a calcium channel blocker, meaning it works by preventing calcium from entering the muscle cells of the uterus. Without enough calcium inside those cells, the muscle fibers can’t contract effectively. This is the same class of drug used for high blood pressure, but in pregnancy it’s repurposed to quiet the uterus.
Nifedipine is taken by mouth, which makes it practical and easy to administer. In most treatment protocols, the initial dose is 10 mg, repeated every 15 to 20 minutes if contractions continue, up to a maximum of 40 mg in the first hour. After that acute phase, the typical maintenance approach is 20 mg every 6 to 8 hours for two to three days. Nifedipine is generally well tolerated, though it can cause flushing, headache, and a temporary drop in blood pressure.
Indomethacin: Effective but Time-Limited
Indomethacin is an anti-inflammatory drug that works by blocking the production of prostaglandins, hormone-like substances that promote uterine contractions. By cutting off prostaglandin production, indomethacin can be highly effective at quieting preterm labor.
The catch is that prostaglandins also keep a critical blood vessel in the fetus open: the ductus arteriosus. This vessel allows blood to bypass the baby’s fluid-filled lungs during pregnancy, and it needs to stay open until birth. Because indomethacin can cause this vessel to constrict prematurely, its use is restricted to pregnancies under 32 weeks of gestation. Even within that window, treatment is typically kept to 48 hours or less, since fetal side effects, including reduced amniotic fluid and changes in heart function, become more likely with prolonged use. When indomethacin is used, fetal heart monitoring with echocardiography is often recommended to watch for signs of vessel narrowing.
Terbutaline: Limited by Safety Concerns
Terbutaline belongs to a class of drugs that relax smooth muscle by activating specific receptors on uterine cells. It was once a mainstay of preterm labor treatment, but its role has narrowed significantly. The FDA issued a boxed warning (the most serious type of drug safety alert) stating that injectable terbutaline should not be used beyond 48 to 72 hours in pregnant women because of the risk of serious maternal heart problems, including abnormal heart rhythms, fluid in the lungs, and death.
The FDA also specified that terbutaline should not be used in outpatient or home settings. It may still be given as a short-term injection in a hospital when a clinician judges it necessary for an urgent situation, but it is no longer considered appropriate for ongoing or repeated use to prevent recurrent contractions.
Magnesium Sulfate: Primarily for Brain Protection
Magnesium sulfate has a long history in obstetrics and is still sometimes listed among tocolytics, but its role has shifted. While it does have some ability to reduce contractions, its most important use in preterm labor today is fetal neuroprotection. Five randomized controlled trials and multiple large analyses have shown that when magnesium sulfate is given before a preterm delivery, it significantly reduces the baby’s risk of cerebral palsy at age two.
The protective mechanism isn’t fully understood, but magnesium appears to shield developing brain cells from damage caused by inflammation and overactivation of certain receptors. It’s typically given intravenously with a loading dose over 15 to 30 minutes, followed by a continuous low-dose infusion until birth or for up to 12 to 24 hours. So while you may see magnesium sulfate mentioned alongside tocolytics, it’s most accurately described as a neuroprotective agent that happens to have mild contraction-reducing properties.
Atosiban: Available in Europe, Not the U.S.
Atosiban works by a different mechanism than the other options. It directly blocks oxytocin receptors on the uterus, preventing the hormone oxytocin from triggering contractions. This targeted approach means it tends to have fewer systemic side effects than drugs that act on the heart, blood vessels, or other organs. Atosiban is approved and used in many European countries for preterm labor management, but it has not been approved by the FDA and is not available in the United States.
Why the Goal Is 48 Hours, Not Full Term
No tocolytic medication has been shown to prevent preterm birth entirely or meaningfully improve long-term neonatal outcomes on its own. What they can do is delay delivery long enough for corticosteroids to take effect. The benefit of corticosteroids peaks between 2 and 7 days after the first dose. During that window, the steroids accelerate the baby’s lung maturation and stimulate surfactant production, a substance that keeps the air sacs in the lungs from collapsing.
This is why the American College of Obstetricians and Gynecologists recommends that tocolytic therapy be reserved for pregnancies where the fetus would genuinely benefit from a 48-hour delay. If the pregnancy has reached a gestational age where the baby would do well without corticosteroids, or if there’s a medical reason delivery needs to happen immediately, tocolytics are not used.
When Tocolytics Should Not Be Used
There are situations where stopping contractions would be more dangerous than allowing labor to proceed. These include:
- Intrauterine infection (chorioamnionitis): Delaying delivery while an active infection is present puts both the mother and baby at serious risk.
- Severe preeclampsia or eclampsia: When the mother’s blood pressure is dangerously high with organ involvement, delivery is the treatment.
- Significant placental bleeding: Placental abruption or other causes of heavy bleeding require delivery rather than delay.
- Fetal distress: If monitoring shows the baby is not tolerating labor, prolonging the pregnancy could cause harm.
- Lethal fetal abnormality or fetal death: Tocolysis offers no benefit in these circumstances.
Each tocolytic also has its own specific contraindications. Nifedipine should be avoided in women with very low blood pressure or certain heart conditions. Indomethacin is avoided after 32 weeks and in women with kidney problems or bleeding disorders. Terbutaline is contraindicated in women with heart disease. The choice of which tocolytic to use depends on the gestational age, the mother’s health profile, and the clinical setting.