Pregnancies between 24-34 weeks of gestation using nifedipine and MgSO4 for tocolysis will be evaluated. Preterm labor is defined as persistent uterine contractions (at least two contractions every 10 minutes or four contractions in 1 hour) resulting in cervical changes (at least 80% effacement and 2 cm cervical dilation). Nifedipine will be administered as an initial 10 mg oral loading dose every 20 minutes for one hour, followed by 10 mg oral maintenance doses every 6 hours for 48 hours. The MgSO4 treatment protocol involves a 100 mL infusion of 5% dextrose containing 4-6 grams of MgSO4 (MgSO4 15% amp.) as an intravenous (IV) loading dose over 20 minutes, followed by a maintenance dose of 2 grams per hour IV. During treatment, patients will be monitored hourly for urination patterns, deep tendon reflexes, and respiratory rate. Treatment will be stopped 6 hours after the cessation of contractions. Doppler measurements will be taken before the first treatment dose and 48 hours after.