Oxytocin in Multiparous Women

Induction of labor (IOL) is defined as an artificial initiation of uterine contractions before the spontaneous onset of labor. It is indicated when maternal or fetal risks associated with continuation of pregnancy outweigh the risks associated with early delivery. Augmentation of labor refers to the stimulation of spontaneous but inadequate contractions. In high-income countries, up to 25% of all deliveries at term involve IOL.

Bishop score is a scoring system which measures changes in the cervix. It is based on several characteristics such as cervical effacement, dilatation, consistency, position and fetus head station. A Bishop score < 6 is often referred to as an unripe cervix (unfavorable), whereas ≥ 6 is referred to as a rip cervix (favorable).

Oxytocin is a commonly used drug for IOL, especially in a favorable cervix. It is a mammalian neuro-hypophyseal hormone and is used to generate regular coordinated contractions originating from the fundus towards the cervix. It is administered intravenously (IV) as an increasing infusion, titrated to the strength and frequency of uterine contractions.

There is voluminous experience with oxytocin, and it is considered as a safe drug. Although, there are several case reports reporting rare serious side effects such as: iatrogenic hyponatremia, grand mal seizure and coma, and maternal death. In a prospective observational study of 287 women at term, demonstrated that hyponatremia was correlated significantly with fluid infusion during labor, and not to oxytocin administration.

There is conflicting evidence whether oxytocin infusion should be continued during the active phase of labor or not. Prolonged administration of oxytocin may increase the risk of tachysystole and uterine rupture, which may result in fetal non-reassuring heart rate and caesarian deliveries. Conversely, discontinuing oxytocin infusion after a few hours may have an effect on time to delivery interval, duration of rupture of membrane (ROM) and chorioamnionitis.

one study had shown in an in vitro study that continuous exposure of human myometrial cells to oxytocin led to a loss of responsiveness to oxytocin. Another study, examined myometrial cell cultures from women undergoing cesarean delivery (emergent and elective). They showed that pretreatment of the cells with oxytocin resulted in a decrease in the percentage of myometrial cells that responded to subsequent oxytocin exposure. They found that preliminary exposure of 4.2 hours to oxytocin yielded half-inactivation to the second exposure.

the investigators have previously investigated whether oxytocin infusion duration increases 24-hour delivery rates and affects time to delivery length and patient's experience in nulliparous women. The investigators' results showed that continuous infusion of oxytocin for labor induction in nulliparous women with a favorable cervix may be superior over intermittent oxytocin infusion, since it shortens time-to delivery, decreases chorioamnionitis rate and improves maternal satisfaction, without affecting adverse maternal or neonatal outcomes.

In this study, the investigators' aim is to investigate whether continuous administration of oxytocin is superior to intermittent administration also in multiparous women, in terms of time to delivery and the percentage of women delivering within 24 hours.