A When the latent phase reaches 18 hours in nulliparous women, the likelihood of successful vaginal delivery decreases markedly.
This paper explores 2 sides of the same question:
- When has an induction failed?
- Is there an optimal length of the latent phase where the vaginal delivery rate is high enough without placing the mother or baby in significant jeopardy?
What constitutes a “failed” induction?
As the authors point out, we lack an exact definition. One group of researchers developed a definition based on outcomes.1“In their frame-work,” Simon and Grobman note, “a failed induction of labor may be diagnosed in women whose continued lack of progression into the active phase makes it unlikely that they would safely proceed to a vaginal deliv-ery.” The investigators1 opined that, in nulliparous gravidas, a latent phase of up to 12 hours was safe, while longer periods carried a low chance (13%) of vaginal delivery.
Simon and Grobman performed their study to “further determine the most clinically relevant definition of a failed induction of labor.”
Details of the study
This was a relatively small retrospective chart review of 397 nulliparous women who were induced for medical or elective reasons. Of these, 32% underwent prior cervical ripening with the use of an extraamniotic saline-infusion catheter for 6 hours. The latent phase began with the initiation of oxytocin and amniotomy and ended when either 4 cm cervical dilation and 80% effacement were achieved, or the cervix dilated to 5 cm regardless of effacement. Only 2% of women never achieved active labor prior to cesarean section, but the rate of cesarean delivery increased in near linear fashion with the lengthening of the latent phase. Nevertheless, 64% of women who had a latent phase up to 18 hours delivered vaginally. After 18 hours in the latent phase, the rate of vaginal delivery dropped such that the women who had a latent phase of 18.1 to 21 hours had a cesarean rate of 69%.
Other risks of a prolonged latent phase
Maternal hazards were an increased risk of chorioamnionitis and postpartum hemorrhage, though this did not translate into a lengthened hospital stay or increased transfusion rate. There was no appreciable neonatal consequence of a prolonged latent phase as measured by meconium, special care nursery admission, or umbilical cord pH.
This study provides some reassurance that, when the latent phase is 18 hours or less, patience may pay off with a vaginal delivery and acceptable maternal and neonatal risk. Keep in mind, however, that this study did not address the role of misoprostol for cervical ripening. Nor was it powered to assess the risk for relatively rare outcomes such as hysterectomy.
The commentators report no financial relationships relevant to these articles.