Comparison of the Effects of Traction Catheter and Normal Saline Amniotic Infusion on Cervical Ripening in Primiparous Women with A Bishop Score Less Than Four

EXTENDED ABSTRACT

Background

Labor induction is the stimulation of uterine contractions before spontaneous labor and is used when the expected maternal or fetal benefit of delivery exceeds the benefit of continuing pregnancy [1]. Elective induction for convenience alone is not recommended because induction in nulliparous women with an unfavorable cervix may be associated with a higher cesarean-delivery risk [2, 3]. Indications for induction include several maternal and fetal conditions, whereas contraindications generally parallel those for vaginal birth [4, 5]. Cervical status is a major determinant of induction success. The Bishop score provides a practical method for evaluating cervical readiness, and a score of 4 or less is commonly regarded as an unfavorable cervix requiring ripening before induction [7]. Cervical-ripening methods are broadly pharmacological or mechanical [8, 9]. Mechanical approaches include transcervical balloon catheters and extra-amniotic saline infusion (EASI) [10, 11]. Reviews have suggested that mechanical methods can provide effective ripening with fewer maternal and fetal adverse effects than some pharmacological methods, while also being relatively simple and inexpensive [11–13]. Previous studies have reported beneficial effects of EASI and balloon-based techniques, but comparisons among specific mechanical strategies remain inconsistent [14, 17]. The present trial therefore compared a traction transcervical catheter with EASI for cervical ripening in primiparous women with a Bishop score below 4 who were candidates for pregnancy termination.

Methods

This single-blind randomized clinical trial was conducted at the obstetrics and gynecology service of Ayatollah Mousavi Hospital, Zanjan, Iran, during 2021-2022. Eligible participants were primiparous women aged 18-35 years with a gestational age of 37-42 weeks, a Bishop score below 4, a singleton fetus in cephalic presentation, intact membranes, and willingness to participate. Exclusion criteria were previous uterine or cervical surgery, active vaginal bleeding, ruptured membranes, placenta previa, known fetal anomaly, intrauterine growth restriction, cephalopelvic disproportion, or another contraindication to vaginal delivery. Sample size was calculated from the study by Mei-Dan et al. [17], and 70 women were enrolled, with 35 assigned to each group. Allocation was performed by block randomization using blocks of four and a random-number table. In the EASI group, 30 mL of normal saline was instilled into the cervical catheter balloon, followed by extra-amniotic normal saline infusion at 1 mL/min. In the traction-catheter group, a 24-F transcervical catheter was passed through the internal cervical os under direct visualization, the balloon was filled with 30 mL sterile saline, and the catheter was connected to a urine bag containing 500 mL saline that was suspended from the bedside to provide traction. After spontaneous catheter expulsion, the Bishop score was reassessed and oxytocin was started when adequate uterine contractions were absent. The participant was blinded to the assigned intervention because patient satisfaction was an outcome. Outcomes included time from oxytocin administration to full dilatation, duration of the second stage, mode of delivery, patient satisfaction, maternal complications, neonatal birth weight, Apgar scores at 1 and 5 minutes, and NICU admission. Analyses were performed with SPSS version 26 using independent-samples t tests, Mann-Whitney tests, chi-square or Fisher exact tests, and Wilcoxon tests as appropriate, with P<0.05 considered significant. The study had ethical approval from Zanjan University of Medical Sciences (IR.ZUMS.REC.1400.502) and was registered in the Iranian Registry of Clinical Trials as IRCT20220403054399N1.

Results

Seventy women were analyzed, 35 in the EASI group and 35 in the traction-catheter group. The groups did not differ significantly in maternal age, gestational age, maternal body mass index, or neonatal birth weight (all P>0.05). The overall mean maternal age was approximately 27.4 years. NICU admission occurred in 3 neonates (8.6%) in the EASI group and 5 (14.3%) in the traction group, without a statistically significant difference (P>0.999). Apgar scores at 1 and 5 minutes were also similar between groups. The interval from catheter placement to spontaneous expulsion and the interval from catheter expulsion to delivery were both significantly shorter in the traction group than in the EASI group (both P<0.001). The median duration of the active phase was 341.4 minutes in the EASI group and 311.70 minutes in the traction group, a statistically significant difference (P=0.035). Mean duration of the second stage was 51.62±6.46 minutes with EASI and 54.12±7.29 minutes with traction, with no significant difference (P=0.13).

Baseline Bishop scores were not significantly different between the groups (P=0.542). After the intervention, the Bishop score was significantly higher in the traction group than in the EASI group (P=0.03). Within-group comparison also showed significant improvement from before to after intervention in both the EASI group (P=0.043) and the traction group (P=0.021). Cervical dilatation at catheter removal was 3.1±1.8 cm in the EASI group and 3.8±1.5 cm in the traction group; effacement at catheter removal was 40.10±1.2% and 40.11±9.5%, respectively. Neither dilatation nor effacement differed significantly between groups (P=0.765 and P=0.786, respectively).

Table 3. Comparison of Bishop scores before and after intervention in the two study groups.

Maternal safety outcomes were similar between the interventions. There were no statistically significant between-group differences in uterine atony (P=0.36), uterine hyperstimulation (P=0.71), cesarean delivery (P=0.71), or the need for stimulation (P=0.63). Patient satisfaction, however, favored the traction method: 40% of women in the EASI group reported satisfaction compared with 74.2% in the traction group (P=0.002). Dissatisfaction in the EASI group was mainly attributed to the sensation of wet bedding, whereas in the traction group it was more often related to pain. These findings indicate that both mechanical methods achieved cervical ripening without a detectable difference in short-term maternal or neonatal complications, but the traction catheter produced a higher post-intervention Bishop score, shorter labor-related intervals, and greater maternal satisfaction.

Table 4. Frequency of selected maternal outcomes in the EASI and traction groups.

Conclusion

Both EASI and a traction transcervical catheter were effective mechanical methods for cervical ripening in primiparous women with Bishop scores below 4. The traction approach was associated with a significantly higher post-intervention Bishop score, a shorter active phase and catheter-related intervals, and substantially higher patient satisfaction, while maternal complications, cesarean delivery, neonatal Apgar scores, birth weight, and NICU admission were comparable between groups. The study therefore supports the traction catheter as the preferred method in this population. Interpretation should consider the single-center, single-blind design and the absence of long-term maternal or neonatal follow-up described by the authors.

Keywords: Bishop Score, Cervical Ripening, Labor Induction

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