The physiology of uterine contractions

Understanding the physiology of the uterus during term and preterm parturition is important for developing methods to control uterine function and is essential to solving clinical problems related to labor. To date, only inaccurate and subjective methods have been used to assess gestational changes in uterine function. To quantitatively evaluate the uterine activity, the authors in the past several years developed noninvasive methods based on recording of uterine electrical signals from the abdominal surface uterine electromyography (EMG), a method that allows reliable and immediate assessment of uterine contractility. Studies in animals and humans indicate that uterine performance can be successfully monitored during pregnancy using EMG. EMG might be used to better define management for a variety of conditions associated with labor. The potential benefits of the proposed instrumentation and method include: reducing the rate of preterm delivery, improving maternal and perinatal outcome, monitoring treatment, decreasing cesarean section rate, and providing research methods to understand uterine function. Labor is the physiologic process by which a fetus is expelled from the uterus to the outside world and is defined as regular uterine contractions accompanied by cervical effacement and dilatation [1]. For normal labor at term, biochemical changes in the cervical connective tissue usually precede uterine contractions and cervix dilatation. Preterm labor, defined as labor before 37 weeks of gestation-the most common obstetrical complication-occurs in about 20% of pregnant women. In the United States alone, 10% of the four million neonates born each year are premature [2,3]. At $1500 a day for neonatal intensive care, this constitutes a national-health-care expenditure well over $5 billion [4]. In addition, preterm labor accounts for 85% of infant mortality and 50% of the surviving infants' neurologic disorders. Current tocolytic therapy has not decreased the rate of preterm delivery. It is argued that the failure of the current strategies to decrease the rate of preterm labor may be because of the fact that, once preterm labor is diagnosed, any therapeutic benefit is lost or temporary. Therefore, one of the key's to treating preterm labor would be early detection or prediction. Recently, the authors developed a noninvasive method for the objective evaluation of the status of the uterus. It consists of recording EMG signals from the abdominal surface. Because action potentials are responsible for contractility of the uterus, recording of EMG activity can be used to assess the contractile function of the uterus. This method has been validated in animals, and preliminary studies in humans support its clinical use. EMG may supplement or replace the methods used currently to assess uterine function (tocodynamometry, intrauterine pressure catheter).