 D. Benefit to Risk Ratio There is no tocolytic agent approved by the FDA and more importantly there is no drug used for this purpose that is free of maternal and fetal side effects. However, any treatment involves less risk than a preterm delivery. Because of long experience in the medical community, magnesium sulfate remains the number one choice of obstetricians throughout the United States, but it has a rather high rate of maternal side effects often leading to discontinuation of the drug. For acute treatment of preterm labor both calcium channel antagonists as well as anti-prostaglandins appear be more effective while having equal or better safety profiles for the mother and baby when used appropriately. The benefits of effectively prolonging pregnancy for several weeks far outweigh any medication effects to the mother or fetus and therefore, make continued investigation in these drugs reasonable. 