Tocolytics Trial: Intravenous (IV) Magnesium Versus Oral Nifedpine in Fetal Fibronectin (FFN) Postive Population

Inclusion Criteria: * Patients between 240/7-320/7 weeks gestation with findings consistent with preterm labor defined by at least one of the following criteria in accordance with regular uterine contractions (\>4 uterine contractions in 20 minutes): 1. Cervix \> 2 cm in dilation or 80% effaced 2. Positive fetal fibronectin (if performed). 3. Demonstrated cervical change between two exams within 90 minutes. Exclusion Criteria: * Negative fetal fibronectin * \> 5cm dilatation * Multiple gestations * Known fetal anomalies or chromosomal abnormalities * Ruptured membranes * Significant vaginal bleeding * Suspected chorioamnionitis * Preeclampsia or uncontrolled hypertension * Non-reassuring fetal heart tracing * Placenta previa and/or accreta * Placenta abruption * Intrauterine growth restriction * Maternal renal disease * Underlying maternal cardiac condition * Symptomatic hyperthyroidism * Significant maternal disease * Contraindication to nifedipine or magnesium * Cerclage presence * Tocolytic use within the last 12 hours * Hypotension (defined as average blood pressure of \<70/40's unresponsive to 1000 cc fluid bolus