Cervical status is a good predictor of the likelihood of vaginal delivery when labor is induced. Any induction method is likely to be effective in a woman with a favorable cervix, whereas no method is highly successful when performed in a woman with a cervix that is unfavorable (ie, firm, posterior, and neither dilated nor effaced). Therefore, if the cervix is unfavorable, a ripening process is generally employed prior to induction.
Cervical ripening is a complex process that results in physical softening and distensibility of the cervix, ultimately leading to partial cervical effacement and dilatation [1-3]. Remodeling of the cervix involves enzymatic dissolution of collagen fibrils, increase in water content, and chemical changes (figure 1). These changes are induced by hormones (estrogen, progesterone, relaxin), as well as cytokines, prostaglandins, and nitric oxide synthesis enzymes.
The two major techniques for iatrogenic cervical ripening are (1) mechanical (physical) interventions, such as insertion of catheters or cervical dilators, and (2) application of cervical ripening agents, such as prostaglandins. These techniques will be reviewed below. General issues regarding induction of labor and use of oxytocin are discussed separately. (See "Principles of labor induction".)
The Bishop score is a quantitative means of describing cervical status prior to induction (table 1). Although thresholds vary among studies, there is general agreement that a score >8 indicates a favorable cervix (ie, the probability of vaginal delivery is the same whether labor is spontaneous or induced), while a score ≤6 generally defines an unfavorable cervix (ie, the probability of vaginal delivery is lower if labor is induced) .
LIMITATIONS OF AVAILABLE DATA
Interpretation of available data on cervical ripening is hindered by several factors, which make it difficult to determine the methods of cervical ripening and labor induction, either used alone or in combination, that are most effective. These factors include: