Labor that occurs after 20 weeks and before 37 weeks of pregnancy is referred to as preterm labor. The causes may be maternal, fetal, or placental factors. Maternal factors include diabetes, pre-eclampsia, abdominal surgery, trauma to the abdomen, cervical incompetence, and infection (especially urinary tract infection - UTI). Fetal factors include multiple pregnancy, hydramnios (excess amniotic fluid), and infection. Placental factors include placenta previa. Other cases reveal a strong correlation between preterm labor and low socioeconomic status (education, income, occupation) and/or history of preterm births.
Drugs for Preterm Labor
Drugs to help stop preterm labor (tocolytics) include terbutaline sulfate (Brethine), indomethacin (Indocin), nifedipine (Procardia), and magnesium sulfate.
Terbutaline is a selective beta receptor stimulator which relaxes uterine smooth muscle. Although it may be used to stop labor, it is not FDA approved for this use (however, it has been approved by the FDA for the treatment of asthma since it also relaxes smooth muscles of the small airways of the lungs).
The longer terbutaline is used, the less effective it is. Therefore, it is normally used to delay labor for 24 to 48 hours. This will help allow time to treat certain causes (like an UTI), to give time for corticosteroids (like betamethasone) to help mature fetal lungs, or to get a mother to a hospital that provides intensive care for neonates.
Side effects from terbutaline can be serious to mother and fetus. They can include tachycardia (fast heart rate), palpitations, dysrhythmias, chest pain, and cardiac arrest (mothers with a known heart condition should not take terbutaline). Other side effects include anxiety, tremors, headache, insomnia, dizziness, nausea, vomiting, and fever.
Indomethacin is a nonsteroidal anti-inflammatory drug (NSAID) used to treat arthritis. It slows down uterine contractions by blocking the production of prostaglandins. Like the drug terbutaline, indomethacin is also used for short term treatment of preterm labor.
Side effects from indomethacin appear fewer on the mother compared to other tocolytic medications but it can be more serious on the fetus (if the treatment lasts 7 days or longer). Indomethacin can cause the fetus’s kidneys to produce less urine, causing oligohydramnios (deficiency of amniotic fluid) and may also affect blood circulation. Side effects on the mother are rare and may include headache, nausea, vomiting, dizziness, heartburn, and stomach upset.
Nifedipine (another drug used for short term treatment) is a calcium channel blocker used to treat hypertension and angina. Besides relaxing blood vessels throughout the body, it also relaxes uterine muscles. According to small studies, nifedipine may help stop preterm labor better than other drugs and cause fewer problems with the newborn.
Side effects can include headache, flushing, nausea, dizziness, tremors, muscle cramps, and hypotension.
Magnesium sulfate is an anti-convulsant drug that is commonly used to treat pre-eclampsia (high blood pressure, fluid retention, and protein in the urine during pregnancy). It is believed to stop labor by affecting the action of calcium (which must be present for the uterus to contract). Results from studies show that it is unlikely to stop labor and may cause complications to both mother and fetus. It is also used for short term treatment.
Side effects include headache, nausea, vomiting, muscle weakness, flushing, blurry vision, slurred speech, chest pain, slowed breathing, and pulmonary edema (fluid in the lungs). Side effects normally occur with the loading dose and decrease over the duration of treatment.
WebMD: Preterm Labor - Medications - https://www.webmd.com/baby/tc/preterm-labor-medications
Linda Skidmore-Roth, Mosby’s Nursing Drug Reference (2000)