FRIDAY, June 29, 2007 (HealthDay News) -- The largest study of its kind has found that magnesium sulfate, the drug traditionally used to delay preterm labor, has more side effects than nifedipine, a newer treatment.
The effectiveness was about equivalent, suggesting that nifedipine is a viable alternative to the older drug.
"It's OK to use nifedipine," said study author Dr. Deirdre Lyell, an assistant professor of obstetrics and gynecology at Stanford University Medical School. "Physicians have had concerns. They've been more comfortable with magnesium sulfate, but one can be comfortable with nifedipine as well, and it has added benefits."
"Most doctors have been using magnesium sulfate, and nifedipine is the new drug, so they are trying to make people more comfortable with the thought of using an alternate drug to magnesium sulfate," added Dr. Jennifer Wu, an obstetrician/gynecologist at Lenox Hill Hospital in New York City.
Preterm labor is defined as onset of labor before 37 weeks' gestation. When it's not possible to prevent premature birth, physicians try to delay delivery for at least 48 hours so they can transfer the mother to an appropriate medical facility and prepared the fetus for birth.
Some 12.3 percent of births in the United States are preterm, a rate that continues to rise.
Treatments called tocolytics, including magnesium sulfate and nifedipine, appear to relax the muscles of the uterus and stop changes in the cervix that lead to delivery. It has been unclear, however, which is superior. Nifedipine is an anti-hypertensive drug.
Many physicians have been more comfortable with magnesium sulfate, partly because this is what they trained to use and what they are familiar with, Lyell said.
The study, which is published in the July issue of Obstetrics & Gynecology, involved 192 women in preterm labor who were randomly assigned to receive magnesium sulfate (intravenously) or nifedipine (orally).
Magnesium sulfate was more effective in preventing delivery for 48 hours without uterine contractions but no more effective in other outcomes, including actual delivery time.
"They speculate that nifedipine may work in a different way, rendering contractions less effective," Wu said. "Magnesium sulfate seems to quiet the contractions down."
Newborns whose mothers had received magnesium sulfate were more likely to be admitted to the neonatal intensive care unit, although the reasons for this were unclear.
But two-thirds of women receiving magnesium sulfate experienced mild to severe side effects, such as shortness of breath and fluid build-up in the lungs.
Only one-third of those receiving nifedipine experienced side effects such as headaches.
"When you're considering medication, you need to consider the risks, and you certainly have a lot more mild and severe side effects with magnesium sulfate," Wu said. "One of the big risks is fluid in the lungs, which can be quite serious. Nifedipine seems to have fewer side effects, but we need to make sure the patient's blood pressure is relatively stable. If blood pressure drops too quickly, the patient can suffer a heart attack, but most pregnant women in labor don't have blood pressure issues," Wu added.
More information
There's more on preterm birth at the March of Dimes.
