Because the Cook Cervical Ripening Balloon (CRB) is indicated for mechanical cervical dilation prior to induction, we’re sometimes asked about the difference between labour induction and cervical preparation. Many physicians say that when you decide to intervene by preparing the cervix, you’ve started the induction.
Labour is a continuum of overlapping phases rather than a series of discrete steps, but preparation of the cervix always precedes initiation of uterine contractions. Preparation of the cervix before induction is often referred to as preinduction.
In spontaneous labour, before the contractions start, the cervix begins to ripen. The term cervical ripening refers to the process by which the cervix softens, thins, dilates, and becomes more elastic.
The amount of ripening present is factored into the patient’s Bishop score. The Bishop score is a 13-point scale that takes into account five parameters:
- The amount of cervical effacement
- The amount of cervical dilation
- The consistency of the cervix
- The position of the cervix
- The position of the fetal head
The Bishop scoring system helps to predict the patient’s cervical readiness. If the amount of cervical ripening is low, then the patient’s Bishop score will be lower. And a cervix with a low Bishop score is termed unfavourable because it is not adequately prepared for vaginal delivery. The lower the Bishop score at the time that uterine contractions are initiated, the higher the probability that the delivery will end in a caesarean section.1
Although “[t]here is no universally accepted definition of favorable or unfavorable cervix,” physicians often consider a Bishop score of less than 6 as an indication of an unfavourable cervix that will need to be prepared for induction.2
When a Bishop score is low, a mechanical or pharmaceutical intervention is considered in order to help improve the score, thereby improving cervical readiness.
The CRB works by applying consistent pressure to both sides of the cervix throughout the dilation process. According to Dr. Jack Atad, the inventor of the double-balloon catheter, “Gradual mechanical dilation is achieved by the pressure [that is] applied on both the external and the internal os due to the inflated balloons.”3
The CRB can be left in place up to 12 hours, and studies have shown high success rates in improving the patient’s Bishop score within that time frame.3 After the CRB is removed, the induction proceeds according to the facility’s protocol.
How induction is defined, either as initiation of uterine contractions or initiation of contraction plus cervical preparation, will vary from individual to individual. But regardless, cervical preparation and the stimulation of contractions are distinct components of a successful induction.
- Vrouenraets FP, Roumen FJ, Dehing CJ, et al. Bishop score and risk of cesarean delivery after induction of labor in nulliparous women. Obstet Gynecol. 2005;105(4):690-697.
- Grobman W. Techniques for ripening the unfavorable cervix prior to induction. UpToDate Web site. https://www.uptodate.com/contents/techniques-for-ripening-the-unfavorable-cervix-prior-to-induction. Updated July 17, 2018. Accessed July 19, 2018.
- Atad J, Hallak M, Ben-David Y, et al. Ripening and dilatation of the unfavourable cervix for induction of labour by a double balloon device: experience with 250 cases. Br J Obstet Gynaecol. 1997;104(1):29-32.