Active labor -- when your cervix has started to dilate -- is very difficult to stop. But if you're in preterm labor, having contractions more than three weeks before your due date, your doctor might prescribe tocolytic medications to slow or stop the process. Tocolytics can decrease the risk of delivery within the next 48 hours, buying time for administration of corticosteroids to help mature the infant's lungs before birth, according to medical practice guidelines from the American College of Obstetricians and Gynecologists.
Terbutaline is classified as a beta-sympathomimetic drug, and it helps to relax smooth muscle. Because the uterus contains smooth muscle, this drug can prevent and stop uterine contractions. Terbutaline is available in oral, injectable and intravenous formulations. For safety reasons, however, the FDA has warned against use of terbutaline tablets for any type of tocolysis. The injectable form may be used during premature labor, but for no longer than 72 hours under close hospital supervision. Potential side effects of terbutaline include a rapid heartbeat in both mother and fetus, maternal low blood pressure, tremors, heart palpitations, low potassium levels, shortness of breath or chest discomfort and high blood glucose levels in a pregnant woman. Terbutaline is classified as a Class C drug in pregnancy, meaning that animal studies have shown signs of undesirable fetal effects from use.
Magnesium sulfate slows uterine contractions by decreasing neuromuscular transmission. Magnesium sulfate also acts as a central nervous system depressant and also dilates blood vessels, lowering blood pressure. The American College of Obstetricians and Gynecologists Committee on Obstetric Practice reviewed available studies on the use of magnesium sulfate to reduce the risk of cerebral palsy in infants whose mothers were in preterm labor. The Committee concluded that use of magnesium sulfate before anticipated preterm delivery reduces the risk of cerebral palsy in the preterm infant. Magnesium sulfate can be given by mouth, via injection or intravenously. The FDA recommends using magnesium sulfate in pregnancy for no longer than 5 to 7 days, because longer use can cause demineralization of the baby's bone and possible fractures. For this reason, it is classified as a category D medication in pregnancy, meaning adverse effects have occurred in humans. Side effects in pregnant women include muscle weakness, flushing, headache, lethargy, nausea and respiratory depression. In infants, this drug can cause poor muscle tone, lethargy and respiratory depression.
Calcium channel Blockers
Calcium channel blockers such as the drug nifedipine reduce the passage of calcium into smooth muscle, such as the muscle of the uterus and the heart. Calcium channel blockers may offer the best outcomes of the commonly used drugs to stop preterm labor, delaying delivery for 7 days, the February 2010 American Family Physician" reports.
Like magnesium sulfate, calcium channel blockers can cause flushing, headache, nausea and low blood pressure in a pregnant woman. Dizziness and problems with maternal heart rate can also occur. There are no known side effects in the fetus, however. Nifedipine is taken orally and is classified as a Class C drug in pregnancy.
Nonsteroidal anti-inflammatory medications, more commonly known as NSAIDS, can slow or prevent preterm labor by blocking production of prostaglandins, which help cause uterine contractions. To stop preterm labor, the NSAID indomethacin is given intravenously, by mouth or via rectal suppository. Side effects are more severe for the fetus than the mother with use of indomethacin. NSAIDS can cause constriction of the ductus arteriosus, a blood vessel that shunts blood away from the lungs before birth and closes to allow oxygen to reach the lungs after birth. NSAIDS can also cause pulmonary hypertension, or increased pressure in the blood vessels that carry blood from the heart to the lungs. In a pregnant woman, NSAIDS can cause gastrointestinal side effects such as nausea, vomiting and reflux. Indomethacin shouldn't be used to treat preterm labor after week 32 of pregnancy, a February 2010 article in "American Family Physician" advises. Indomethacin is a Class C drug during pregnancy.
Talk to your doctor about the risks and benefits before taking any new medications during pregnancy. If you notice signs of preterm labor, whether or not you're taking medication to prevent it, call your doctor immediately. Signs include experiencing more than four contractions in an hour or leaking fluid from the vagina. Report contractions even if you aren't sure that what you're feeling is strong enough to be considered labor.