Induction of labor is the artificial stimulation of uterine contractions before spontaneous onset of labor for the purpose of accomplishing vaginal birth.¹ The goal of labor induction is to achieve vaginal delivery.²

Before 41 0/7 weeks’ induction of labor should generally be performed based on maternal or fetal indications. After 41 0/7 weeks’ induction of labor should be performed to reduce the risk of cesarean and the risk of perinatal morbidity and mortality.³

Augmentation of labor is the stimulation of uterine contraction when spontaneous contractions have failed to result in progressive cervical dilatation or descent of the fetus.⁴ Since uterine activity is characterized by frequency, intensity, and duration of contractions, it may be desirable to augment these forces in a patient who is in labor but not progressing adequately.

Oxytocin is a hormone, a peptide consisting of nine amino acids that is synthesized by the hypothalamus then transported to the posterior pituitary gland where it is released into the maternal circulation in a pulsatile fashion. Oxytocin is released in response to breast stimulation, cervical stretching, and stimulation of the lower genital tract. Oxytocin released in response to vaginal and cervical stretching results in uterine contractions.⁵ Oxytocin is the most common pharmacologic agent used for both induction and augmentation of labor in the United States.⁶ It is also the hormone responsible for the “let down” of milk from alveolar cells in the breast during the postpartum period.

Oxytocin increases free intracellular calcium, which is essential for smooth muscle activity. To exert its effect, oxytocin must bind with the oxytocin receptors. Oxytocin receptors in the uterus increase throughout gestation to reach their maximal levels at term.⁷ In fact, oxytocin receptors at term are increased 300 times over nonpregnant level. Additionally, the increase of actual receptors is also accompanied by an increase in uterine responsiveness to oxytocin at term.^3,8 It is important to note that although oxytocin is an effective induction agent for women with a favorable cervix, it is not effective as a cervical ripening agent.⁹

The pharmacokinetic half-life of oxytocin is generally accepted to be between 10 and 12 minutes.^10,11,12,13 Three to four half-lives of oxytocin are needed to reach steady-state plasma concentrations.¹¹ Uterine response to IV oxytocin administration occurs within 3 to 5 minutes of IV administration, and within 40 minutes a steady-state plasma concentration is achieved.¹⁴ This information led Seitchik et al. to recommend at least a 40-minute interval between increases of oxytocin to allow time for the oxytocin to reach a steady state and exert its full effect on the uterus.¹⁵ This dosing regime is intended to prevent women from receiving higher doses of oxytocin than are necessary.¹⁵ Risks associated with oxytocin are generally dose related and the most common side effect is tachysystole. While oxytocin is the most common medication used for labor induction and augmentation, it is also the drug most commonly associated with preventable adverse events during childbirth.¹⁶

Tachysystole is now the preferred term to describe excessive uterine activity. Tachysystole is defined as:

NOTE: Other terms like hypertonus and hyperstimulation are not well defined and should be avoided.

Indications for induction of labor are not absolute and should take into consideration maternal and fetal conditions, gestational age, cervical status, and other factors.¹ The following are examples of maternal and fetal conditions that may be indications for induction of labor:

Cervical ripening is the process of effecting physical softening and distensibility of the cervix leading to effacement and dilatation in preparation for labor and delivery.⁶ A variety of different methods may be used to induce cervical ripening. These include mechanical devices and several different prostaglandin preparations. Before beginning any cervical ripening or induction, assessment of the cervix should be done and a Bishop score assigned. Since cervical status is the most important factor predicting the success of an induction, this score helps to identify those women who would benefit from cervical ripening before beginning an induction of labor.

A Bishop score is a method of evaluating how ready or “ripe” the cervix is for induction.³⁰ A cervical examination is done to evaluate dilatation, effacement, consistency (i.e., softness or firmness), and cervical position, as well as station of the presenting part. The findings are scored on a scale that was developed based on studies of many women undergoing labor induction (Table 8.1). Inductions are more likely to be successful when a woman’s cervix has undergone certain biochemical changes. Based on the assessment findings and using a scoring system for these changes, it identifies which women are good candidates for labor induction.