Those studied are females 18 years old or older, between 24-34 weeks gestation, have a reassuring fetal heart rate (FHR) tracing, and in preterm labor as defined by the American College of Obstetrics and Gynecology (ACOG). The participants must have persistent uterine contractions, with 4 every 20 minutes or 8 every 60 minutes, and any one or more of the following: documented cervical change, > 1 cm cervical dilation, or > 80% cervical effacement. The exclusion criteria are the same as for most tocolytic therapy; most importantly, premature rupture of the membranes (PROM), and additionally, severe preeclampsia, severe abruption placenta, chorioamnionitis, fetal death, fetal anomaly incompatible with life, severe fetal growth restriction or estimated fetal weight (EFW) <5%, mature fetal lung studies, maternal cardiac arrhythmias, a permanent cardiac pacemaker, a fetal cardiac arrhythmia, or any other contraindication for tocolysis.