An ultrasound prediction model for probability of vaginal delivery in induction of labour

Objective

Our aim was (1) to evaluate a pre-induction ultrasound score for prediction of vaginal birth and compare it with the Bishop score in term nulliparous women, and (2) to formulate a prediction model to calculate probability of vaginal delivery for clinical use.

Methods

Ninety six nulliparous women between 36-41 weeks gestation were recruited. All subjects fulfilled the inclusion criteria of a live singleton pregnancy, vertex presentation, intact amniotic membrane, in the absence of active labour with no contraindication to vaginal delivery. The patients were assessed by our ultrasound score comprising of 3 cervical and 2 fetal head parameters. These parameters were fetal head position, fetal head symphysis pubis distance relation, cervical length, funnelling and posterior cervical angle. Each parameter was scored from 0-2,with a maximum score of 10.A second obstetrician blinded to the sonographic findings assessed the modified Bishop score. SPSS 20 was used for ROC curves plots and calculation of area under curve. Binary Logistic Regression model was prepared and probability of vaginal delivery for various scores was calculated.

Results

Out of 91, 61(67%) achieved active phase of labour and 54(59%) had vaginal delivery. Our pelvic ultrasound score showed better sensitivity and specificity in comparison to the Bishop score. At a cut off of ≥ 5, the ultrasound score showed sensitivity of 79.3%, specificity of 75.8%, whereas, the Bishop score showed sensitivity of 66.7% and specificity of 44.2%. Binary logistic regression model predicted 78.0% of the events correctly.

Conclusion

Our study shows that “Garg Ultrasound Score” can predict success of induction of labour in nulliparous women . This proposed pelvic ultrasound score, if validated in larger multicentre studies, could help clinicians provide evidence-based counselling for predicting probability of vaginal delivery. This in turn, may allow women make a more informed decision before undergoing induction of labour.
 

Keywords

Induction of Labour, Bishop Score, Garg Ultrasound Score, Induction Success, Prediction Model

Introduction

Labour is a complex physiological process, and we have limited understanding of factors that initiate the process of labour. Induction of labour is an obstetric intervention undertaken when continuation of pregnancy is thought to be associated with maternal or fetal risk, with an aim to have a vaginal birth. While it is one of the most common obstetrical procedures [1,2], our ability to predict success of induction is limited.
 
Induction of labour has been found to have a major impact on the birthing experience of women.In this era of positive birthing and shared decision making, this needs serious introspection. It shows that clinicians need better tools to predict success in induction of labour, so that they can customize their counseling based on individual patient charactertics [3,4] . Since 1941, many scoring systems have been suggested and till date bishop score is universally the most accepted one. However, this is a subjective method with high inter and intra observer variability, uncomfortable and less precise.[5,6,7,8] So obstetricians need an objective, painless, easy and precise method.
 
In the last 30 years, lot of work has been done to evaluate the role of ultrasound in prediction of success of induction. Sonographic cervical length was the first ultrasound parameter to be studied , however the results are conflicting with no consensus on appropriate cut off value[9]. In recent years’ various other ultrasound parameters like posterior cervical angle, funneling, wedging pattern, fetal head position, fetal head perineum distance, fetal head pubis symphysis distance have been evaluated either individually or in combination and compared with Bishop score. [9,10,11]
 
Many authors formulated scoring systems based on combination of ultrasound and clinical parameters and compared with Bishop score. [12,13,14] Inspite of large volume of work and acceptability of superiority of ultrasound over digital examination still we have not been able to put it into clinical practice. This has prompted us to design and study GARG scoring system based on five ultrasound parameters – 3 cervical and 2 fetal head. This is in consonance with Bishop score which has 4 cervical and 1 fetal head parameter. We have selected these parameters based on results of previous studies and have been found to be easily measurable and have shown favorable correlation in prediction of successful induction: Cervical length, posterior cervical angle, funneling, fetal head position, fetal head symphysis pubis relation.

The present study is to compare the five parameters of modified Bishop score [15] and five parameters of GARG ultrasound score to predict successful labour induction. Our aim was (1) to evaluate a pre-induction ultrasound score for prediction of vaginal birth and compare it with the Bishop score in term nulliparous women, and (2) to formulate a prediction model to calculate probability of vaginal delivery for clinical use.

Methods

This was a quasi-experimental study which included 96 women admitted for induction of labour at Chaitanya hospital, Chandigarh, India between December 2017 to Jan 2019. Inclusion criteria was singleton nullipara between 36-41 weeks with live fetus in vertex presentation with intact amniotic membranes and no signs of labour. Exclusion included ; multiple pregnancy, malpresentation, previous scarred uterus, abnormal placentation, fetus with congenital anomalies, fetal intrauterine death, cervical encirclage in present pregnancy were excluded. All women gave consent to participate in the study. This study received approval of internal ethics committee of hospital.
 
On admission a detailed history was followed by general and systemic examination. The gestational age was reconfirmed based on the date of last menstrual period and ultrasound measurement at or before 12 weeks of gestation. All ultrasounds were performed by one investigator using Voluson S6 Ultrasound machine. Immediately before induction and bladder emptying determination of the fetal occipital position and fetal head symphysis pubis relation (FHPR) was done by trans abdominal ultrasound. A transvaginal sonography was done to assess the cervical funneling, cervical length and posterior cervical angle. A printout of all ultrasounds was taken and measurement was done using scale and protractor and recorded in performa. Subsequently a second obstetrician blinded to the sonographic findings assessed the Bishop score and findings were recorded in a separate performa.
 
Sonographic measurements of cervical length, posterior cervical angle, fetal head position were measured according to Rane et al., funneling was according to Chung et al. For Fetal Head –Symphysis Pubis Relation (FHPR) determination the convex probe was kept vertically on pubic region so as to visualise the symphysis pubis and fetal head simultaneously. A line perpendicular and starting from superior margin of symphysis pubis was drawn toward fetal head and checked if the distance between this line and fetal head was measurable , not measurable or touching (as shown in Figure 1(a,b,c).Each parameter was scored from 0-2, with maximum score of 10 (Table 1). Modified Bishop score was assessed and scored as per Table 2.

For induction of labour cervical ripening was done with buccal Tab misoprostol 25 mcg, maximum of three doses at 4 hourly interval until uterine contractions reached a frequency of three in 10 mins. Oxytocin if required was started in escalating doses. Continuous electronic fetal heart rate monitoring and tocodynamometry was used in all women. Success of induction of labour was defined as vaginal birth. Failed induction was defined as inability to achieve active phase of labour corresponding to cervical dilatation of ≥ 5 cm within 8 hours of initiating oxytocin. Non-progress was defined as no cervical dilatation after active phase of labour for at least 2 hours and/or no decent of fetal head during second stage of labour for at least 2 hours despite of adequate uterine contraction. Failed induction, non-progress of labour and fetal distress were considered as an indication for cesarean delivery.

Descriptive Analysis was done in form of numbers, percentages, mean and standard deviation represented in form of tables and figures. Student t test and Pearson Chi square test was applied to check significance of results. Sensitivity, Specificity, False Positive Rate and Correct Classification Rate(Accuracy) were calculated for vaginal delivery. ROC curves were plotted for most suitable cut off points 4, 5 and 6. The data collected were analysed using MS Excel 2007 and SPSS 20.

Results

A total of 96 women were enrolled. However, 5 women did not complete the trial .Therefore 91 were included and evaluated (Figure 2). Indications for labour induction were: Cholestasis of Pregnancy (n=24), Gestational Diabetes (n=15), Gestational Hypertension (n=4), Intrauterine Growth Retardation (n=7), and Post Datism (n=23) and Elective (n=18).  Of 91 women, 61 (67%) went into active phase of labour. 54 (88.5%) delivered vaginally and 7 (11.5 %) required cesarean – 5 for non-progress of labour and 2 for fetal distress.  Of 30(33%) women who did not achieve active phase of labour and underwent cesarean – 15 had failed induction and 15 developed fetal distress. Regarding mode of delivery, 54(59.4%) out of 91 enrolled women achieved vaginal delivery and 37 (40.6 %) required cesarean - 17 for fetal distress, 15 for failed induction and 5 for non-progress of labour.
 
The characteristics of 91 patients are given in Table-3.The mean age of enrolled women was 29.87±3.29 years (Range 21-40 years). Mean BMI was 29.09±4.43 (Range 19.7– 46.1). Mean gestation was 269.2±7.96 days (Range 256 - 282 days). Table 4 gives comparison of sensitivity, specificity and other diagnostic parameters at different cut offs of 4, 5 and 6. Figure 3 gives comparison of ROC curves for prediction of mode of delivery.
At cut off value of 5 for Garg ultrasound score   [sensitivity -79.3.0%, specificity-75.8%] and cut off  4 for Bishop Score [sensitivity – 69.0%, specificity-55.6%] produces best combination. ROC was better for Garg Ultrasound Score than Bishop Score with AUC .855 in comparison to AUC of .622 (Table 5) Garg Ultrasound Score shows highly significant  P value. A model of binary logistic regression, with equation formula :probability = 1/1 + e-x, was designed that included total Garg Ultrasound Score as variable.

The following equation was obtained: P =1[1 + EXP(-5.739-1.294*USGS)] where P is the probability of  Vaginal Delivery and USGS is total Garg Ultrasound Score obtained. Values equal or greater than 0.5 predicted vaginal delivery. The model predicted 78.0 % of the events correctly (Table 6).
Table 7 shows calculated probabilities for each calculated value of Garg Ultrasound Score so that obstetrician can use it easily in practice. Neonatal Outcomes are Shown in Table 8. Mean baby weight at the time of delivery was 2957 gm (1890-3870 gm). Mean APGAR Score at 1 minute was 7.8 (4-9) and at 5 minutes was 8.9 (6-9) . Out of 91 babies, 82 (91 %) were shifted to mother and only 8 (9 %) required specialized neonatal intensive care. None of our enrolled women developed complications associated with induction of labour like uterine hyper stimulation and rupture of uterus. There was no maternal and neonatal mortality.
 
 

Discussion

This study shows that GARG Ultrasound Scoring System has sensitivity similar to Bishop Score in prediction vaginal birth (79.3% vs 69%). However GARG ultrasound scoring system has a clearly significant higher specificity (75.8% vs 55.6%) for vaginal birth and hence is superior to Bishop Score. Though our ultrasound score system has shown statistical significance at scores of 4, 5 and 6; we propose cut off of 5 since it has shown a combination of maximum sensitivity and specificity and accuracy.

The success of induction depends on favorability of cervix and fetal head position which till date is assessed by manual examination and scored as Bishop score. However, being clinical, it has shown to be inherently subjective with high inter and intra observer variability and variable sensitivity and specificity.[17] To overcome this limitation, a number of researchers have studied accuracy of one or more Ultrasound parameters either alone or in combination with Bishop Score and/or maternal characteristics for prediction of successful IOL. .[6,10,13-15,18-20] However, none of these have so far gained widespread acceptability and clinical application because of conflicting results. The five parameters of Ultrasound scoring system proposed by us have individually shown to have good accuracy in various studies. Out of these 5 parameters – 3 assess cervical favorability and 2 assess fetal head position and station in relation to pubis symphysis.

Ultrasound measurement of cervical length has been the most studied single parameter which has been compared with Bishop Score for prediction of successful IOL. Due to conflicting results in its prediction of success and wide variation in cut off points when used alone is not useful in prediction of vaginal delivery. Our scoring system has five parameters including cervical length and has shown statistically significant results. A number of authors have studied and a few have proposed scoring systems consisting of a combination of ultrasound parameters, one or more Bishop Score parameters and/or maternal characteristics. Rane et al and Keepanasseril et al have shown good sensitivity and specificity but their results cannot be compared to our study because their patient population included multiparous women. We feel that maternal characteristics should not be included in scoring system since they remain unchanged regardless of whether the patients are induced or come in spontaneous labour. Similarly, the scoring system proposed by Eggebo et al includes digitally measured dilatation of cervix which again brings the limitation of subjectivity and patient discomfort.
 
Neha Bajpai et al (2015) formulated ultrasound scoring system with parameters which matched the components of Burnet modified Bishop Score including cervical length, funnel length, funnel width, position of cervix and distance of presenting part to external os. At cut off of 4 , they achieved sensitivity of 64.52% and specificity of 85.71% for Bishop Score and sensitivity of 77.42% and specificity of 92.86% of ultrasound score respectively for prediction of entering into active phase of labour. However this scoring system parameters especially funnel length and width have difference of only 5 mm which adds subjectivity to the measurement. Their study population includes both nullipara and multipara with more number of multiparous women.
 
The strengths of the Garg score include –(1) it has five parameters which take into account cervical as well as fetal head status thus providing a more complete assessment similar to Bishop Score. (2) We have well defined protocol for measurement of each parameter. All these parameters are easily measurable, do not require high degree of skill, can be performed quickly and provide an objective assessment. (3) Ultrasound is well tolerated and hence more acceptable to patients as well as clinicians. (4) Our prediction score can be handy for obstetrician in clinical practice.

The potential limitations of the present study are (1) small sample size so validation through larger multicenter study is required (2) studied in only nulliparous women before IOL – we need to test diagnostic accuracy in other groups like multiparous women, spontaneous labour, V-BAC and preterm labour (3) Cervix undergoes changes with increasing gestation, our study included patients from 36-41 weeks, so better to convert cervical length into MOMS. (4) Vaginal birth depends on fetal head circumference and maternal pelvic parameters like inter ischial diameter and subpubic angle.[21] Inclusion of these parameter’s may improve the accuracy. (5) Confounding variable effect  i.e epidural analgesia need to be evaluated. (6) Our cesarean rate is high (40%) in comparision to the expected cesarean rate of 25-30% according to Robson classification . (7) Use of misoprostol as induction agent can affect the results due to its association with fetal distress .So we need to validate Garg ultrasound score in women using other induction agents (foleys , dinoprostone ) .

Our study has shown high sensitivity and specificity, and we suggest that Garg Ultrasound Scoring System has an excellent scope of being used in clinical practice for prediction of success of IOL. It can help clinicians in providing evidence based counseling to pregnant women for informed shared decision making.
 

Conclusion

Through this study, we propose Garg Ultrasound Scoring System which can predict success of induction of labour with greater specificity as compared to the traditional Bishop Score. This Scoring System is highly objective, reproducible, easy to perform and does not include digital vaginal examination which is subjective and painful for patients. This scoring system, if validated in more diverse population in larger multi-centric studies, clinicians should be able to provide individualized counseling and help women to make  more informed decision.

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	File/Dsecription
	Figure-1 Fetal Head - Symphysis Pubis Relation (FHPR)
	Figure-2 Study enrollment
	Figure-3 ROC Curve according to mode of delivery
	Table-1 Ultrasound score
	Table-2 Modified Bishop Score
	Table-3 Baseline demographic characteristics of enrolled women
	Table-4 Diagnostic characteristics of Bishop Score and Ultrasound Score in predicting Mode of Delivery
	Table-5 Area Under Curve in ROC Analysis according to mode of delivery
	Table-6 Predictive Model
	Table-7 Calculated Probabilities for each Garg Ultrasound Score
	Table-8 Neonatal outcomes