 Preterm birth is the most common and costly complication in obstetrics. It complicates up to 11 of all pregnancies and it is responsible for 70 of sick babies. The ideal way to stop preterm labor when it occurs which drug to use is not known. Currently magnesium sulfate is used by about 95 of all practitioners, but recent data suggest magnesium given this way may be harmful for the babys future development. Other drugs such as antiprostaglandin agents are very effective in stopping uterine activity, but particularly when used for 48 hours have been associated with both maternal and fetal sides effects. Lastly, calcium channel antagonists are effective in stopping contractions and have very little in the way of maternal and fetal side effects, but less data is available in the United States on their use. Because there is no FDA approved drug to stop preterm labor, we purpose to randomize all women with preterm labor 20-34 weeks to receive one of the above three methods of stopping preterm labor. The primary outcomes will be to see which agent stops the uterine contractions most effectively, for the longest period of time with fewest relapses and results in significant prolongation of pregnancy. If one of these agents is clearly superior to the other two it would help women avoid early delivery or have significant extension of their pregnancy to avoid some of the complications of preterm birth in the baby. 