 Electrical intervention EI uses bipolar, constant-current 1-20 mA, square-wave pulses in 20 duty cycles. Women in active preterm labor have an electrode catheter placed into the posterior fornix of the vaginal canal. The EI current is given up to 80 minutes while monitoring tocodynamometric toco contractions and adjunct electrohysterographic EHG activity while continuously monitoring maternal vital signs, fetal heart rate and electrocardiogram fECG. The study includes a pre-EI control period C1; the EI period, when a 10-second current burst is delivered only during a contraction; and a post-EI control period C2. The whole study will take a maximum of two hours. 