Annals of SBV
Volume 12 | Issue 1 | Year 2023

Utility of Ultrasonographic Parameters for Monitoring of Progress of Labor and Prediction of Successful Vaginal Delivery: Narrative Review

Papa Dasari

Department of Obstetrics and Gynaecology, Jawaharlal Institute of Postgraduate Medical Education and Research, Puducherry, India

Corresponding Author: Papa Dasari, Department of Obstetrics and Gynaecology, Jawaharlal Institute of Postgraduate Medical Education and Research, Puducherry, India, Phone: +91 9442566883, e-mail:

How to cite this article: Dasari P. Utility of Ultrasonographic Parameters for Monitoring of Progress of Labor and Prediction of Successful Vaginal Delivery: Narrative Review. Ann SBV 2023;12(1):18–25.

Source of support: Nil

Conflict of interest: None

Received on: 04 May 2023; Accepted on: 11 June 2023; Published on: 19 July 2023


Traditionally, progress of labor and the prediction of vaginal delivery is done by clinical assessment. There is a lot of subjectivity in assessment and increased interobserver variability, especially with digital vaginal examination. For the past two decades, many ultrasonographic parameters were developed, and their utility was studied objectively. Of the ultrasound (USG) parameters, head position is the easiest to recognize. The descent and station of the head are determined indirectly by angle of progression (AOP) and head perineal distance (HPD). Cervical dilatation does not correlate well with digital vaginal examination, and it is difficult to visualize cervical rim beyond 8-cm dilatation. Midline angle and head-up position are other parameters in the second stage of labor that are determined to predict difficult vaginal delivery. Angle of progression of >110° and HPD <4 cm are good predictors of vaginal delivery. “Sonopartogram” may replace the traditional partogram as it is more objective, noninvasive, and women-friendly. Intrapartum apps are incorporating USG parameters with other patient characteristics to develop models for predicting vaginal delivery.

Keywords: Intrapartum ultrasound, Monitoring of labor progress, Prediction of vaginal delivery, Sonopartogram.


Ultrasound (USG) examination in obstetrics has become an integral part of antenatal care. Good antenatal care determines the fetuses that are to be carried to term and also diagnoses the problems of the placenta, liquor, and uterine and adnexal lesions and helps in planning the mode of delivery as to whether a particular woman can undergo vaginal delivery or needs a scheduled cesarean section. This approach is necessary in women with complications that account for approximately 15%. The majority of women undergo the process of labor culminating into normal delivery or instrumental delivery, and some require an emergency cesarean section. Labor is the most common emergency, and it is common to encounter women who had antenatal care elsewhere and come to us only for delivery.

The word “intrapartum USG” initially was used to refer to “monitoring of labor by USG (Sonopartogram)”, but off late, it is used in its broader sense to diagnose all problems occurring across all stages of labor, namely, the third stage as well as the fourth stage.1 Hence, the above title is chosen to be specific on which the narrative review is intended.

It is more than two decades since research on “intrapartum USG” has evolved and various guidelines have been put forth. But in practice, labor monitoring exclusively by USG (Sonopartogram) is not practiced though its advantages have been proved by evidence. Women who are uncooperative for vaginal examination, which is unpleasant and painful when repeatedly done for monitoring labor, are forced to undergo the same and at times abused by healthcare workers. This review is intended to increase awareness among obstetricians, nurse midwives, and other paramedical workers who are committed to practicing safe and women-friendly monitoring of labor.


Articles published on “sonopartogram”, “USG labor monitoring”, and “intrapartum USG”, predicting successful vaginal delivery by USG’s “utility of USG in second stage of labor” , prediction of safe instrumental delivery, etc. are analyzed. Systematic reviews, meta-analyses, and guidelines are also studied and the evidence-based recommendations from these are narrated and simplified.


Progress of labor is traditionally monitored by abdominal palpation and digital vaginal examination to know the two important aspects, i.e., cervical dilatation and descent of the presenting part, and graphically represented for interpretation as “Partograph”. It was devised by Dr Emanuel Friedman of Columbia University, who published a study describing his clinical observations of 500 primigravid white women at a single hospital in New York City.2 The curve is known as “Friedman’s curve” and he has determined the length of latent phase, active phase, acceleration phase, phase of deceleration, and duration of second stage of labor by multiple per-vaginal examinations almost every 1–2 hours. Later, the partograph has undergone many modifications, and the frequency of vaginal examinations has been standardized to every 4 hours after establishment of good uterine contractions, and now, WHO partograph is being followed throughout the world, where plotting of partograph begins with the active phase.3 The value of partograph is questioned in the current era of technology where labor is monitored more effectively, but still, it is found to be indispensable in low-resource settings for timely referral and to prevent obstructed labor, thus saving maternal and fetal lives.4 Of late, labor care guide (LCG), which includes respectful maternity care and birth companion and second-stage events, is being introduced, replacing the WHO partograph.5 The time durations of the active phase, acceleration phase, etc., are not defined in LCG as standardized in Friedman’s and other modified partographs, and they are of the opinion that duration of labor can vary in each individual and labor can be allowed as long as maternal and fetal conditions are good and following this statement is not going to help in early referral and prevention of maternal distress and obstructed labor.


It has been reported by various studies that compared to digital examination of labor events with that of USG parameters, it was found that digital vaginal examination is more subjective and is not reproducible, whereas USG parameters of labor monitoring are more objective and reproducible with acceptable interobserver and intraobserver variations. The errors in digital vaginal examination occurred among residents and consultants when assessment was undertaken even on a birth simulator with a sensor as per ACOG stations (–5 to +5). The errors were as high as 50–80% for fetal head position and stations, both among residents and consultants.6

Further repeated vaginal examinations predispose to intrapartum sepsis and the probability of sepsis is reported to be high (4–13%).7 Vaginal examination is uncomfortable for women who are already in pain and has negative psychological sequelae in some. In women with preterm rupture of membranes, vaginal examinations may shorten the latency period of labor. The most important factor is that vaginal examinations are imprecise, and high inconsistent findings can cause distress in women. The World Health Organization (WHO) recommends limiting the number of digital vaginal examinations.8 National Institute for Health and Clinical Excellence (NICE) has recommended further research aimed at reducing the frequency of digital vaginal examinations in normal labor.9

The Components of Labor Monitoring by USG

The pattern of labor monitoring is essentially the same as clinical, except that USG is used objectively to monitor the position of the head, descent of the head, and cervical dilatation, and one should master the parameters to be measured technically and represent them as sonopartograph.10 For this, it is essential to do USG abdominally initially and later transperineally by 2D USG probe.

Head Position and Rotation

A study comparing the determination of fetal head position by clinical examination (digital vaginal examination) and USG on 196 women found that USG could detect head position (anterior, posterior, and transverse) in all cases, whereas digital vaginal examination failed to detect the same in 32%. They concluded that USG is a better method in determining head position and the agreement between the two methods is poor, and the agreement specifically for the occiput posterior position was very low (κ = 0.1).11

A prospective observational study conducted at Manipal University, India, in 2014 compared transabdominal USG with digital vaginal examination for determination of head position. Consultants and residents were included in performing the examinations. The point of examination was ≥3 cm dilatation as determined by standard vaginal examination in singleton pregnancies with cephalic presentation in women in labor between 35 and 41 weeks of gestation. The degree of agreement between the two examination methods was analyzed by using Cohen’s Kappa test. Kappa value <0.2 was considered to be poor. Fetal head position as determined by vaginal examinations was consistent with those obtained by USG only in 31.5%. Cohen’s Kappa test of concordance indicated a poor concordance of 0.15. There was no significant difference in measurements performed by residents and consultants.12

Zahalka et al. compared transabdominal USG, transvaginal USG, and digital vaginal examination (DVE) to determine fetal head position in the second stage of labor. They found that transvaginal USG accurately determined the positions of the head than TAS and DVE. This is because it is difficult to see the orbits of the fetus in a deeply engaged head by TAS. Discrepancy was >90° in 15% and 20% of cases, respectively (p < 0.02 for comparison of TAS and TVS).13

Fetal head position will not remain the same throughout labor, internal rotation determines the fetal head position that varies with time. The most favorable position to predict vaginal delivery is occipitoanterior, and unfavorable positions are occipitoposterior and occipitotransverse, but these unfavorable positions become favorable by undergoing rotation during the course of labor. To find out these facts whether USG can be used to determine the head position during the course of labor, Adam et al.14 observed the fetal head behavior by USG from the beginning of labor till the second stage. They did the study on 187 women of gestational age >37 weeks from the beginning of labor. They found that none of the fetal head positions that were anterior at the beginning of labor rotated posteriorly. Majority of those (89.2%) with occipitotransverse and occipitoposterior at the beginning of labor became anterior and had vaginal birth, and 10.8% were in persistent occipitoposterior and required cesarean section. These findings were in contrast to the initial observations reported by Gardberg et al.15 in 1998, where they found that 68% of occipitoposterior positions developed from the initial anterior positions and only 32% were occipitoposterior at the beginning of labor as determined by USG. Increased operative intervention was required in women where the fetal head position was persistent occipitoposterior from the beginning of labor. To resolve the controversy if occipitoposterior delivery is the consequence of persistence of an initial occiput-posterior position or malrotation from an initial occiput-anterior or transverse position, Akmal et al.16 performed a prospective study using TAS in 918 women at term in active labor at various cervical dilatations. They found that the incidence of occipitoposterior position was 33% at 3–5 cm of dilatation, 33.9% at 6–9 cm, and 19% at 10 cm. They concluded that the majority of occipitoposterior positions become anterior during the course of labor by 10 cm dilatation, and the majority of persistent occipitoposterior positions are due to occiput being posterior at the beginning of labor rather than anterior positions malrotating to posterior and persist as posterior.17

It is important to recognize the malposition fetal head position at full dilatation, as the labor outcomes were reported to be poor for both mother and baby. The maternal adverse outcomes include increased incidence of instrumental, cesarean deliveries, severe perineal lacerations, and postpartum hemorrhage.18 A retrospective cohort study reported a higher incidence of Erbs’s and facial nerve palsy in fetuses born as occipitoposterior than those delivered from the OA position. Vaginal delivery from the persistent OP position is associated with increased maternal morbidity, and operative vaginal delivery from this position is associated with increased neonatal morbidity.19

It is well-known that forceps-assisted deliveries are associated with anal sphincter injuries, but the association is high in fetal head malposition such as occipitoposterior position. In a retrospective cohort study, it was found that the odds of having sphincter injury were 2.2.20 This is also true for vacuum-assisted deliveries.21

The evidence accumulated by various studies is that fetal head position as determined by digital vaginal examination is inaccurate and the inaccuracy ranges from 20 to 70%, and USG is the standard (evidence level-I). For the head rotation also, many studies have demonstrated the superiority of USG when compared with digital vaginal examination (evidence level-I).22

The technique is simple, and the fetal head position can be easily recognized by placing the transducer on the symphysis pubis and tracing the junction of spine and occiput or by recognizing the fetal orbits. When fetal orbits are seen anteriorly, it is occipitoposterior position.

Cervical Dilatation

Cervical dilatation in the latent phase can be easily obtained by endovaginal (transvaginal) USG. But for plotting a sonopartogram, the active phase of labor starting at 4 cm dilatation transperineal USG with 2D probe is used, which does not cause discomfort for the patient. The probe is placed at the introitus in-between the labia after covering with a sterile glove. The cervix is visualized as a ring of echogenic enhancement with central anechoic amniotic fluid, which represents a bag of membranes (Fig. 1).

Figs 1 A and B: (A) Transabdominal USG shows the cephalic presenting part with cervical dilation. (B) Transperineal USG shows cervical dilatation:cervix seen as a rim of echogenic area

In 2013, a prospective observational study conducted at Skane University Hospital, Lund, Sweden, compared transperineal 2D USG with digital vaginal assessment for cervical dilatation in 86 women in labor. The mean difference between the two methods was 0.9 cm. Cervical dilatation was successfully measured by USG in 71%. The intraobserver repeatability of USG measurements was high.23 The reasons for USG measurement in 71% may be due to the fact that as the cervix becomes dilated more and more, it becomes thin and it is less visible and this is also mentioned in the recent publication by Hassan et al. that the application of transperineal USG for measurement of cervical dilatation is limited in advanced labor and it is applicable upto <8 cm.24 Dimassi K et al. determined the reliability of transperineal USG with that of digital vaginal examination for cervical dilatation performed by a different examiner who was blinded on 56 women. There was good agreement between the two methods as the Pearson correlation coefficient was 0.93.25

A cross-sectional study conducted in 2016 in a teaching hospital in Ghana on 195 women with the aim of detecting active labor by USG and finding out the agreement between USG and digital vaginal examination concluded that there was a good agreement [Pearson correlation coefficient (r) was 0.78 (95% CI 0.72–0.83)] between the two methods and it concluded that USG can be used to detect active labor (≥4 cm) instead of digital vaginal examination.11

A systematic review of 10 observational studies of 856 women aimed to investigate the reliability of USG method compared with digital vaginal examinations in detecting cervical dilatation. It estimated poor correlation between the two methods, the intraclass correlation coefficient r was 0.32 (combined for nulliparous and multiparous women). The correlation coefficient was 0.676 (95% CI 0.419–0.833) for multigravida and it was 0.349 (95% CI 0.25–0.43) for nulliparous women. They concluded that the utility of transperinal USG for nulliparous women needs further study.26 This may be due to the fact that different investigators with various levels of experience were involved in these studies that were pooled together and the measurement of cervical dilatation by digital examination was highly subjective. None of the studies, however, interpreted arrest of dilatation as measured by USG as one of the labor abnormalities as classified by Freidman, but the same definitions that were applied for arrest of dilatation can be used when the dilatation is measured by USG and that can be shown by plotting on a sonopartogram.

Descent of Head and Station of Head

This is one of the most important parameters to know whether labor is progressing normally. Ischial spines denote “0” station and sacral curvature above the spines is divided into five parts of 1 cm each and it is denoted as –1 just above the spines to –5 at the brim. Below the ischial spines, the station is denoted as +. +1 –1 cm below the ischial spines and +5 at the introitus or at the coccyx posteriorly. The examination of determining the station digitally by vaginal examination is very painful for the patient and though a landmark of ischial spine is there, again, lot of subjectivity is reported in the determination of station and thus interpreting whether the head is descending or there is an arrest of descent. The presence of large a caput may also add to the error.

Determination of the station of the head is highly subjective by digital vaginal examination, and this was proved by Dupuis and et al. by conducting a study on mannequins as per AICOG. The error was 30% for residents and 34% for consultants, and errors were maximum for determination of midpelvic station.6 Hasty cesarean section for high stations/unengaged head should not be advocated as it was found that vaginal delivery was as high as 73%.27 Ultrasound (USG) parameter of the station and interpretation of descent is made possible by measuring the angle between the pubic bone and the fetal skull. This is measured after getting the image of the fetus and pelvis in the sagittal section by transperineal scan and freezing the image. A line is drawn along the long axis of the pubic bone and another line drawn tangentially down to the fetal skull bone, and the angle can be measured between these two lines (Fig. 2). Tutschek et al.28 measured this angle several times throughout labor and given the algorithm to calculate and convert into stations as measured clinically (head station in cm = Angle of progression (AOP) (°) × 0.0937−10.911). One can measure the angle and interpret the station from the following table (Table 1). The angle of progression is also called as “angle of descent”, “the progression distance”, “transperineal USG head station”, or sonographic head station.

Fig. 2: Transabdominal USG shows the measurement of angle of progression (115°)

Table 1: Angle of progression (AOP): Station of head (Tutschek B et al.28)
AOP (0) Station (cm)
 84 −5
 90 −4
 95 −3
100 −2
106 −1
116 0
127 +1
138 +2
148 +3
159 +4
170 +5

To assess the cutoff values for angle of progression, AOP was measured at the beginning of the first stage of labor in 101 women and in the second stage in 66. The cutoff value of AOP that predicted vaginal delivery in both stages was 125°.29 Angle of progression was measured in the late first and second stages of labor among 500 women in a prospective cohort study. Angle of progression was significantly higher in spontaneous vaginal delivery group (with cutoff 123° ± 8.5°) as compared with women delivered by vacuum or by CS (113° ± 10.5°) (p = 0.003).30

In India, a study conducted in 2021 on 200 subjects where AOP was determined by transperineal USG concluded that the probability of CS was nil at an angle ≥116° during the first stage of labor. The cutoff value of AOP for predicting vaginal delivery was 99.6° as determined by ROC. Both primigravida and multigravida women who were in labor at or more than 38 weeks of gestation were included.31

The other indirect USG measurements for assessment of descent and station are head perineum distance (HPD) and midline angle (MLA) and head direction.

Head perineum distance is measured after getting the image with the transducer placed perpendicularly to the fetal skull and the distance between the outer and lowest part of the fetal skull and the perineum. This measurement does not denote the curvature of the sacrum and hence cannot be compared with the station as measured clinically or by AOP. Few investigators, however, compared the stations with HPD and they found that HPD of 35 mm or 36 mm represents “0” station.28,32

Midline angle is the angle formed between the anteroposterior axis of the pelvis and the echogenic line present between the two cerebral hemispheres of the fetus. It is an indirect parameter of head rotation through the maternal pelvis. After excluding occiput posterior cases, they found a MLA ≥45° to correspond to a head station of ≤ +2 cm in 98.6% cases and a rotation <45° to correspond to a head station of ≥ +3 cm in 83.7% cases.2

Head direction denotes the angle between the long axis of fetal head and the long axis of symphysis pubis and is classified as head up (an angle of >30°) and head down (an angle <0°) and horizontal when the angle falls between 0 and 30°.33 This is more useful to predict the success of instrumental deliveries.

Utility of USG in Prolonged First Stage of Labor

Head perineum distance and AOP were compared to predict the time remaining for delivery in women with prolonged first stage of labor diagnosed as per WHO criteria. They found that AOP >110° and HPD <40 mm were both good predictors of vaginal delivery in the prolonged first stage of labor.34 A prospective observational study on 600 Egyptian women was conducted between 2017 and 2018 with the aim to find out transperinal USG as a tool for predicting the progress of labor. They have divided subjects into two groups. Group A 300 subjects with normal progress and 300 with prolonged first stage of labor. They concluded that USG is a valuable tool, and the most important parameters to predict progress of labor and mode of delivery were AOP and HPD combinedly and AOP independently.35

For predicting slow progress of labor, AOP and HPD are more reliable than digital vaginal examination. The cesarean section rate was 82% when HPD was >5 cm and 7% when it was <4 cm. Similarly, at an AOP of <100°, CS rate was 62%, and when it was >110°, CS rate was 12%. The level of evidence was 2+.22

Predicting Successful Instrumental Delivery

The fetal head station is the most important parameter and its assessment at the beginning of the second stage is significantly and independently associated with the duration of the second stage and correlated with the risk of operative delivery in both nulliparous and multiparous women (p < 0.001).36 Perineum-to-skull USG distance (HPD) measurement of ≥40 mm (measured in 659 women) was significantly associated with the occurrence of a difficult extraction based on the composite criterion, after adjustment for parity, presentation type, and fetal macrosomia.37 The cutoff parameters taken in the second stage of labor to predict success are AOP >120°, HPD <2–5 cm, MLA <45°, and head-up sign. Failure of vacuum delivery was experienced if AOP was more than 136°. If HPD was >3.6 cm, difficult delivery and associated perinatal asphyxia were reported. The cesarean section rate was 35% when occipitoposterior position was noted, and HPD was >3.5 cm.

Predicting Vaginal Delivery

For the week before delivery, measurements of estimated fetal weight and progression distance (PD) can be used to forecast the delivery mode as part of a policy for pregnant women with prelabor clinical signs. The main finding of the study was that some clinical and US term measurements were correlated with labor outcome. From the variables measured at 37 and 38 weeks, PD and BMI were significantly and strongly correlated with the delivery mode. Thus, these measurements apparently can be used to forecast the delivery mode when the pregnancy reaches term.38

Shrivastava et al. studied occipitospinal angle by abdominal USG in low-risk primigravid women in the first stage of labor. Patients with lower occipitospinal angle (average 125°) had taken a longer time to deliver the baby compared to those with higher occipitospinal angle >125 and women with <120° had cesarean sections.39

A study was undertaken to evaluate the accuracy of transperineal USG in monitoring labor progress among 40 primigravid women and compared with digital vaginal examination. The correlation was fair for HPD in determining fetal head station, strong for cervical dilatation, and moderate for head position.40

To predict the possibility of vaginal delivery at the time of induction of labor, HPD was measured in 250 women. By using ROC, a cutoff of 5.5 cm HPD had a sensitivity of 93% and specificity of 94%. When HPD was ≤5.5 cm, majority had vaginal delivery and only 7.1% required cesarean section. If HPD was ≥5.5 cm, 95% required cesarean section.41 A study to evaluate the repeatability of the measurements of head direction, angle of the middle line, and PD and angle of progression in women in the second stage of labor, Molina et al. conducted a prospective observational study using 3D USG and digital vaginal examination. They have concluded that AOP is the most reliable and reproducible parameter.42

Intrapartum App

The first prediction model was developed in 2015, where the maternal characteristics and USG parameters of 269 women were entered, and the likelihood of vaginal delivery and length of labor were calculated. The likelihood between 75 and 90% predicted vaginal delivery.43

Sonopartogram and Its Implementation

The “Sonopartogram” was first introduced in 2014 by Hassan et al. after the proof-of concept study. They demonstrated that acquisition of data for sonopartogram was more successful than the conventional partogram. In this study, simultaneous assessment of cervical dilatation and fetal head descent and rotation were assessed by USG and digital VE10 The model of sonopartogram is represented in Figure 3.

Fig. 3: Sonopartogram adopted from Hassan et al.

Sonopartographic assessment of all important parameters in labor is being undertaken to obtain normograms in unselected nulliparous and multiparous women with fetal cephalic presentation. The differences of parameters in occipitoanterior and posterior positions will also help in prediction of vaginal delivery. After finalizing the normogram, implementation of sonolabor would be undertaken in a phased manner.44 An observational prospective cohort study, which is recruiting 4,000 participants for assessing the progress of labor clinically and by USG, also aims to find out the psychological impact of labor monitoring by USG. The trial ( Identifier: NCT02326077) ends in October 2023.45

Opinion of Midwives and Patients on USG Monitoring of Labor

Wilkinson et al.46 analyzed the views of midwives, women in labor, and women’s panel on intrapartum USG in a pilot study. Women found the nonintrusive nature and accuracy of USG valuable, while midwives were concerned about deskilling and medicalization of birth. All groups felt a graphical representation of labor on a device would be helpful. The viewpoint of midwives and parturient women regarding intrapartum USG and vaginal examination was assessed by a questionnaire. Parturients preferred USG over vaginal examination, whereas midwives continued to prefer vaginal examination even though the majority of midwives could perform transperinal USG.47

Evidence-based Recommendations

The ISUOG guidelines have given the level of evidence and grade of recommendations for the performance of USG parameters. No grade-A recommendations exist in any parameter. Mostly the recommendations are grade-B.

Recommendations for monitoring of labor by USG throughout labor have not been given, but it was advised that USG would be more helpful in cases of slow progress or arrest of labor in the first or second stage and for ascertaining fetal head position and station prior to undertaking instrumental deliveries.

Expert review on a sonopatrogram concluded that the USG assessments can predict duration of labor, likelihood of success of vaginal delivery, and fetal condition at delivery. The models to predict vaginal delivery need external validation before implementing into practice.48

World Association of Perinatal Medicine guideline recommends MLA to evaluate head rotation and occiput-spine angle when dystocia is suspected in the second stage. This guideline recommends USG during all stages of labor to make objective assessments and practice-intended interventions to improve clinical care.1


Intrapartum USG monitoring of progress of labor is more objective, reproducible, and safe when compared with digital vaginal examination. Angle of progression and HPD are the simple parameters that tell us about station of the head and predict vaginal delivery at a cutoff of >125° and <3.5 cm, respectively. Head direction and MLA help to predict successful instrumental deliveries. “Sonopartogram” may replace the conventional partogram in obstetric practice in the near future. Intrapartum USG evaluation is easy and can be performed by residents and midwives in addition to obstetricians.


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