In this study, we compared delivery results and adverse events between the COOK group and the oxytocin group. We found that pregnant women receiving COOK balloons had almost same delivery results and adverse events as SROM pregnant women with a mature cervix. As for baseline characteristics, average age of the COOK group was significantly younger than the oxytocin group. In addition, gestational age of the COOK group was older than that of the oxytocin group. Previous studies have reported that women with advanced maternal age were more likely to have a preterm delivery (22), suggesting a younger gestational age. Therefore, older gestational age of the COOK group may be explained by younger maternal age. In addition, a number of studies have shown that there is a direct positive correlation between birth weight and gestational age (23–25). Hence, it is possible that heavier birth weight of the COOK group may be partially explained by older gestational age.
The rate of mode of delivery was almost the same in the COOK group (18.1%) and the oxytocin group (20.9%). Since both nulliparous women and multiparous women were included in our study, and combined with inclusion and exclusion criteria, our participants belonged to the 2a and 4a groups according to the Robson ten-group classification system. After excluding multiparous women, CS rate was 19.6% (30 of 153) in the COOK group and 22.8% (36 of 158) in the oxytocin group, respectively. Wu et.al conducted a survey in six midwifery institutions of Shanghai, and reported that the CS rate of the 2a group was 14.53% in 2015 and 2.13% in 2017, respectively (26). Our results showed higher CS rate compared with their study, which may have several possible explanations. First, the two-child policy was implemented nationwide from January 1st of 2016 and our study time-span is from January 1st to September 30th of 2016. It is highly likely that most of the women in our study became pregnant before the implementation of this policy. Studies from central China have reported that after the implementation of the two-child policy, the CS rate in nulliparous patients have decreased (27). Therefore, two-child policy may be one possible explanation for the low CS rate in 2017. Second, Shanghai Putuo Maternity & Infant Health Hospital is a specialized grade 2A hospital, and locates in the urban area. In a study by Zhang et.al, they found that the CS rate of the 2A hospital was higher than that of the 3A hospital (28), which may be related to the fact that secondary hospitals are less capable than tertiary hospitals in prenatal testing, diagnosis and management of maternal and fetal complications, evaluation of surgical indications, and supervision. These reasons lead to the implementation of more CS operations without specified medical indications (28).
Some studies have also compared effects of COOK balloons and oxytocin in promoting ripening of cervixes in pregnant women with full-term singleton pregnancy, cephalic fetal presentation and wish to have vaginal delivery. In the study by Fan et.al, the effects of COOK balloons and oxytocin in pregnant women with immature cervix and complete membrane were compared, and no statistically significant differences of maternal age, gestational age, and Bishop score before the use of balloons and oxytocin were observed between the two groups. They reported that the CS rate was significantly different (18.75% vs. 35%, P < 0.05) of the two groups (14). In the study by Han et.al, although there were statistical differences of age and gestational age between the two groups, no statistically significant differences of neonatal weight, cervical Bishop score before labor induction, and labor induction indicators were observed. They also reported that the CS rate was significantly different (20.4% vs. 37.7%, P < 0.05) of the two groups (29). The reason we did not find significant differences of CS rates between the two groups might be that patients in the oxytocin group had a mature cervix. One strength of our study is that we evaluated whether the placement of COOK balloons can achieve same results as SROM pregnant women with a mature cervix. In addition, we found similar CS rate (18.1%) in the COOK group to the studies of Fan et.al (18.75%) and Han et.al (20.4%). Therefore, our study provides an argument for the role of COOK balloons in promoting cervical maturation and reducing CS rate.
According to the "Guidelines for Promoting Cervical Maturity and Induction of Labor in Late Pregnancy", the advantages of COOK balloons are low cost, stable at room temperature, and low risk of excessive contractions compared with PGE preparations. Disadvantages of COOK balloons include the possibility of potential infections, premature rupture of membranes, and cervical damage (13). Duro‑Gómez reported the cost of misoprostol for IOL was lower than that of dinoprostone or COOK balloons, with similar obstetric and perinatal outcomes (15). Fan et.al reported that the success rate of COOK balloons for cervical ripening (100%) was significantly higher than that of oxytocin (79.38%), and the vaginal delivery rate was significantly higher in the balloon group (81.25% vs. 65%, P < 0.05) as well. In addition, the balloon did not increase maternal complications or adverse infant outcomes, such as umbilical cord prolapses, intrauterine infection, uterine hyperstimulation, and placental abruption. The time span of the first stage of labor in the COOK group was significantly shorter than that in the oxytocin group (6.1 ± 1.3 vs. 10.5 ± 3.2 h, P < 0.05). Furthermore, there were no statistically significant differences in the Apgar score, weight and postpartum haemorrhage rates between the two groups (14). In Han’s study, after removing the COOK cervical dilatation balloon, the Bishop score of the experiment group increased from 2.38 ± 0.7 points before placement to 6.15 ± 0.96 points after placement. The vaginal delivery rate of the COOK group was significantly higher than that of the oxytocin group (79.6% vs. 62.3%, P = 0.005). The occurrence rate of chorioamnionitis in the COOK balloon group was significantly higher than that in the oxytocin group (18.4% vs. 5.4%, P = 0.002). There were no significant differences in other adverse event rates such as uterine hyperstimulation, rapid birth, neonatal asphyxia, postpartum haemorrhage, cervical laceration, or placental abruption (29). In the study by Lim et.al, there was no difference in the pain score between the two groups at the beginning of the IOL. However, thereafter, patients in the COOK group had lower pain scores than patients in the PGE group (4.5 ± 2.3 vs. 5.6 ± 2.4, P = 0.044). Women were equally satisfied with both methods and equally likely to recommend their own methods for IOL (17). In our study, there were no significant differences in adverse events between the two groups. Besides, we found there were no differences in terms of first labor time, second labor time and total labor time between the two groups. While heavier birth weight in the COOK group may be explained by older gestational age. Since the ripening procedure of the cervix in the oxytocin group was a physiological procedure, it is reasonable to conclude that use of Cook balloons may be as effective as the physiological procedure in inducing cervical maturity without extra adverse events.
As for other mechanical methods, Sayed Ahmed reported that compared with Foley catheter, use of Cook balloons would result in greater cervical maturity, which can be reflected as a higher Bishop score after balloon expulsion/removal. The duration from balloon insertion to expulsion and then delivery was significantly shorter when using Foley catheter. There were no differences in pain during insertion, pain after insertion, CS rate, post-partum haemorrhage rate and patient satisfaction (30). In the study of Mei-Dan et.al, compared with the COOK cervical mature balloon group, the Foley catheter group had a significantly shorter time from balloon insertion to expulsion (6.9 ± 4.2 vs. 10.1 ± 4.7 h, P = 0.001) and from insertion to delivery (19.6 ± 11.4 vs. 23.4 ± 15.5 h, P = 0.03). There were no significant differences of other outcomes including use of analgesia, episiotomy, operative vaginal delivery, ripening success rate, CS rate, birth weight, 1 min and 5 min Apgar score, pain perception during catheter insertion, hospitalization length and maternal satisfaction (31). Xing et.al reported that single balloon was more effective than double balloons in cervical ripening. The double balloon catheter increased the time from insertion to removal of the catheter and reduced the spontaneous elimination rate of the catheter. All neonatal outcomes in the two groups were comparable (32). Therefore, we may conclude that COOK balloons are more effective than Foley catheters in promoting cervical ripening. The duration from balloon insertion to expulsion and then delivery is significantly shorter when using Foley catheters. Other outcomes as well as CS rate are not different between the two types of catheters.
Mechanical methods mainly apply pressure on the internal os of the cervix, overstretch the lower uterine segment and promote local synthesis and release of endogenous PGE in the cervix, which induces cervical softening and ripening (13, 33). Compared with Foley catheters which have a single balloon, double-balloon catheters can apply stable and mild pressure on both external and internal os (32, 34), and continuously dilate cervical canals mechanically (29). COOK balloons can open the cervix by 2 to 3 cm without contractions (14).
There are several strengths of our study. First, to the best of our knowledge, this was the first study to compare CS rate and number of adverse events in pregnant women under the placement of COOK balloons with pregnant women in physiological conditions. In the oxytocin group, oxytocin was only used for promoting regular contractions, since the cervixes of SROM pregnant women were mature. Therefore, our results may suggest that COOK balloons are able to promote cervix ripening. At the same time, we also compared occurrences of adverse events in the two groups, and found that there were no significant differences in adverse events, which further confirmed the safety of COOK balloons. However, previous researches have reported that the CS rate correlated positively with maternal age , and we cannot rule out the confounding of maternal age on CS rate in the current study. Additionally, Bishop scores before and after the use of balloons were not recorded, therefore the effect of COOK balloons in the change of Bishop scores cannot be directly evaluated.