Induction of labour is the artificial initiation of labour in a pregnant woman after the age of fetal viability but without any objective evidence of active phase labour and with intact fetal membranes. This procedure is increasingly being carried out in obstetric units for varying indications (Baacke 2006; Crane 2006). The need for induction of labour may arise due to a problem in the mother, her fetus or both, and the procedure may be carried out at or before term. Obstetricians have long known that for this to be successful, it is important that the uterine cervix (the neck of the womb) has favourable characteristics in terms of readiness to go into the labour state (Baacke 2006; Edwards 2000). The definition of failed induction of labour has controversies surrounding it but the risks are clear. Because of the risks of failed induction of labour, a variety of maternal and fetal factors as well as screening tests have been suggested to predict labour induction success (Crane 2006). These include certain maternal factors such as parity (the number of times a woman has delivered), height, weight, body mass index, maternal age, Bishop score and its individual components, fetal factors such as birthweight and gestational age, transvaginal ultrasound (TVUS) assessment of the cervix, and biochemical markers including fetal fibronectin (fFN) and insulin-like growth factor binding protein-1 (IGFBP-1).
Description of the condition
During pregnancy, the cervix is a solid and closed organ. As the pregnancy advances towards the time of labour, the cervix undergoes some stages of remodelling in readiness for delivery (Timmons 2010). The first phase of the remodelling is the stage of softening which involves a decline in the tissue tensile strength and this may start in the first trimester (the first 13 weeks of pregnancy). This first stage is usually slow but progressive and requires the progesterone-rich environment to take place. In the weeks preceding spontaneous labour and delivery, the next stage of cervical ripening commences (Timmons 2010; Word 2007). It is only after the cervix has ripened that it can dilate in response to spontaneous uterine contractions. The process of cervical ripening is a complex one that is associated with an increase in the concentration of the hydrophilic (water attracting) glycosaminoglycans and non collagenous proteins (Leppert 1995; Word 2007). This phase of cervical remodelling is therefore very important as this is what gives the cervix the ability to dilate (open up) in response to uterine contractions of labour. It is not very clear what the role of chemicals such as prostaglandins is in the natural ripening process but it is known that administration of prostaglandins or their analogues will lead to ripening of the cervix in a woman with an unripe cervix. Some studies have also shown that the histological features of a naturally ripened cervix are similar to that induced by exogenous prostaglandins (Rath 1993; Uldbjerg 1983; Word 2007). Although induction of labour is an artificial procedure, it tries to, as much as possible, to mimic the physiological process. Therefore, to expect a successful induction of labour, care should be taken to determine if the cervix is ripe. For the unripe cervix, certain agents should be used to ripen the cervix in order to optimise the success of labour induction.
Description of the intervention
Several methods have been used to assess the ripeness of the cervix prior to labour induction and newer methods are being sought. The traditional method is the cervical scoring system described by Bishop, known as the Bishop score (Bishop 1964). This system assesses the position, consistency, effacement, dilatation of the maternal cervix, as well as the station of the fetal presenting part. The maximum score here is 13 and studies have shown that women with a score of nine or more were more likely to have successful labour induction (Baacke 2006). In his modification, Burnett discovered that women with a score of at least six achieved vaginal birth within six hours in 90% of the time, whereas, the course of labour was unpredictable in women with a score less than six (Baacke 2006; Burnett 1966). Some other studies have also considered a score of six or more as favourable for labour induction (Eggebo 2009). Although Bishop described his method to predict the success of labour induction in parous women with cephalic presentation, the system is used today for every proposed induction of labour (Baacke 2006). This original scoring system is simple to perform but several questions have arisen concerning its ability to objectively assess cervical ripening prior to labour induction and so several modifications of the system have been proposed (Baacke 2006; Burnett 1966; Eggebo 2009; Goldberg 1997; Keepanasseril 2012). Some studies have even shown that it is a poor predictor of the outcome of labour (Hendrix 1998). This has led researchers into searching for alternative methods that may be more objective.
Transvaginal ultrasound (TVUS) assessment of the cervix was subsequently introduced to assess pre-induction cervical ripening (Keepanasseril 2007; Pandis 2001; Rane 2004; Rane 2005; Yang 2004). TVUS is able to measure objectively the cervical length, internal os diameter and the posterior angle. This method has been used to predict preterm delivery (Goldberg 1997) and has found a place in the assessment of pre-induction cervical ripening (Crane 2006). Studies have compared the performance of the Bishop score and TVUS cervical assessment in the prediction of the outcome of labour induction and have given mixed results (Crane 2006; Eggebo 2009; Yang 2004). In some studies, TVUS cervical assessment proved superior to the Bishop score (Pandis 2001; Rane 2004), while in others the superiority was not demonstrated (Gonen 1998; Rozenberg 2000; Rozenberg 2005).
Some chemicals related to pregnancy have also been studied for predicting the success of labour induction. Fetal fibronectin is a glycoprotein that is found in the amniotic fluid and choriodecidual interface in high concentrations but leaks into the vaginal secretion prior to the onset of spontaneous labour (Baacke 2006; Mouw 1998; Roman 2004). In his study, Ahner et al noted that women who tested positive to fFN in their vaginal secretion were more likely to deliver within 24 hours (Ahner 1995). The same study also showed that women with both a low Bishop score and negative fFN test had the highest risk of prolonged labour and operative deliveries. Another study has compared the Bishop score and fFN testing and concluded that their performances were similar (Blanch 1996). Ekman et al also documented that a positive vaginal fFN correlated well with cervical ripeness but recommended a quantitative study to determine the threshold value that can be a cut-off point for ripeness (Ekman 1995). Another chemical that has been evaluated is Insulin-like growth factor binding protein-1 (IGFBP-1). It exists in different parts of the body as isoforms depending on its phosphorylation status. The amniotic fluid contains mainly the non phosphorylated isoform while the decidual tissues contain the phosphorylated isoform (Martina 1997; Westwood 1994). Nuutila and his group have evaluated IGFBP-1 to see if its presence in cervical secretion reflects the ripeness of the cervix (Nuutila 1999).
How the intervention might work
The physiologic cervical ripening that predates spontaneous uterine contractions is associated with shortening of the cervix (effacement), opening of the internal cervical os (dilatation) and softening of the cervix (consistency). To be able to assign a Bishop score, a vaginal examination is performed to assess the state of the cervix in terms of its consistency, dilatation, position and effacement as well as the station of the fetal presenting part. Scores are assigned to each parameter and the total score becomes the Bishop score. The higher the score, the more the cervix is ripe and therefore, successful induction of labour is expected. Harrison et al did show that up to 87% of women with a Bishop score of at least seven will deliver within nine hours, whereas only 44% of those with score of four or less will deliver within the same time frame (Harrison 1977). It may be difficult to fully assess the cervical length especially when the internal os is closed as the finger may not reach the part of the cervix beyond the vaginal fornices (Crane 2006). TVUS is able to measure the cervical length and cervical funnelling (representing dilatation), which are changes associated with cervical ripening (Baacke 2006). TVUS is also able to measure the posterior cervical angle and studies have shown that a value of more than 90 degrees predicts successful vaginal birth (Eggebo 2009; Rane 2004). In another study, cervical length assessment by TVUS predicted successful induction of labour (Yang 2004).
Fetal fibronectin exists in the choriodecidual space and the amniotic fluid and its production increases with advancing gestational age. Uterine activity leads to its leakage into the cervico-vaginal secretion even with an intact fetal membrane (Mouw 1998; Ojutiku 2002). It has been suggested that this leakage precedes the onset of labour by about two weeks and may represent the later stages of physiological events before the onset of labour (Garite 1996). Therefore, its detection in the cervico-vaginal secretion may suggest imminent labour. Ahner et al did show that women with intact fetal membrane undergoing induction of labour were more likely to deliver within 24 hours if they had positive fFN (Ahner 1995). IGFBP-1 exists as the phosphorylated isoform in the choridecidual space and leaks into the cervix and vagina with increasing choriodecidual activity (Martina 1997; Westwood 1994). Its role in the prediction of preterm birth has been assessed by different authors. One study assessed it among symptomatic women and documented a positive predictive and negative predictive values of 24% and 86% for delivery before 37 weeks (Cooper 2012). in another study, the positive and negative predictive value for preterm delivery before 35 weeks were 47% and 93% respectively (Elizur 2005).
Why it is important to do this review
Obstetricians, over the years, have known that the pre-induction cervical status is an important determinant of the outcome of induction of labour. Despite the different modifications, the Bishop score remains the most popular way of assessing the cervix for ripeness but its objectivity and ability to predict vaginal delivery have been contested (Baacke 2006). Currently, there is no strong evidence to suggest the most dependable method for assessing pre-induction cervical ripening since different studies give inconsistent findings (Gonen 1998; Pandis 2001). If such evidence becomes available, clinicians will be guided appropriately in order to optimise the outcome of labour induction.