Ultrasound in Labor and Delivery | Article

This chapter should be cited as follows:
Kamel R, Glob. libr. women's med.,
ISSN: 1756-2228; DOI 10.3843/GLOWM.419173

The Continuous Textbook of Women’s Medicine Series – Obstetrics Module

Volume 18

Ultrasound in obstetrics

Volume Editors: Professor Caterina M (Katia) Bilardo, Amsterdam UMC, Amsterdam and University of Groningen, Groningen, The Netherlands
Dr Valentina Tsibizova, PREIS International School, Florence, Italy

Chapter

Ultrasound in Labor and Delivery

First published: October 2024

AUTHORS

Rasha Kamel

Maternal-Fetal Medicine Unit, Cairo University, Cairo, Egypt

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INTRODUCTION

Ultrasound has transformed management of the fetus and obstetric care. More recently, it has become apparent that ultrasound also plays a role in monitoring labor and delivery. The clinical applications of ultrasound in labor have been increasing in significance, from assessment of fetal viability and wellbeing, presentation and position, placental location and diagnosis of placental abruption, estimation of fetal weight to monitoring progress of labor and aiding in the management of complicated cases. The use of ultrasound for assessment of fetal position and station and for the proper selection of patients eligible for operative vaginal delivery has been demonstrated and incorporated into recent guidelines. This chapter focusses on the diagnostic and prognostic values of ultrasound examination in relation to birth, highlighting current research direction and its potential impact on maternity care in the future.

PRETERM BIRTH

Ultrasound has been proven to have a diagnostic and thus a preventive role in management of preterm delivery by screening for cervical length (CL).¹

Women presenting with symptoms suggestive of preterm birth (PTB) need an accurate and urgent cervical assessment as a guide for appropriate management. Digital vaginal examination of the cervix is not recommended in cases with associated rupture of the membranes due to increased risk of infection and the possibility of shortening the latency period.²^,³ Transvaginal ultrasound assessment of the cervix has been found to be safe and of good predictive value in these cases.⁴^,⁵^,⁶ Moreover, digital assessment of CL is quite subjective and has been recognized for many years to be non-specific.⁷ Indeed, interobserver variability has been reported to be as high as 52%.⁸ Several studies compared digital vaginal examination to ultrasound assessment of the cervix, both in singleton and multiple pregnancies, showing superiority of ultrasound.⁹^,¹⁰ Therefore, transvaginal ultrasonographic measurement is now considered to be the most accurate method for measuring the cervix.¹¹ and guidelines for proper ultrasonographic assessment of CL have been published.¹²

A meta-analysis conducted to assess CL as an independent predictor for preterm birth reported different cut-offs for different gestational ages.¹² Ultrasound assessed CLs have been incorporated into PTB management algorithms aiming to individualize proposed management schemes for maximal cost-effectiveness.

Stratifying by parity and obstetric history adjusted the performance of CL and fetal fibronectin (fFN) levels as screening modalities. In nulliparous women, either quantitative fFN (but with a lower cut-off of 20 ng/mL) or CL assessment can be utilized. In the case of parous women, fFN is not useful and CL assessment should be the main screening tool.¹²

When assessing the risk for inevitable PTB, other ultrasound parameters for cervical assessment have been explored. Transvaginal ultrasound assessment of uterocervical angle, when combined with CL, was reported to increase the predictability of preterm birth.¹³ Cervical funneling has also been evaluated as a PTB risk indicator, albeit, with contradictory findings. Funneling was reported not to be a significant contributor to PTB risk stratification when used independently.¹⁴ However, other groups concluded that funneling was a significant risk factor for adverse perinatal outcome (preterm labor, chorioamnionitis, abruption, rupture of the membranes, and severe neonatal morbidity and mortality) when reported categorically.¹⁵ Using both CL and funneling, the cervical index ((funnel length + 1)/CL) can be calculated. It has been reported that a mid-trimester cervical index of <1 and between 1 and 2 is associated with an increased risk of PTB at <37 weeks and <34 weeks respectively.¹⁶ An additional ultrasonographic parameter for PTB is the presence of ‘sludge’. This term is used to describe the presence of intra-amniotic debris, which has been demonstrated to be associated with an increased risk for PTB before 34 and 32 weeks’ gestation, regardless of CL.¹⁷

ULTRASOUND BEFORE INDUCTION OF LABOR

Induction of labor (IOL) is currently one of the commonest obstetric interventions with a reported incidence of 20%.¹⁸^,¹⁹ Ultrasound can be used to determine the eligibility of a woman for IOL. Before IOL, it is recommended to use ultrasound for proper assessment of gestational age and amniotic fluid volume, and for placental and umbilical cord location and for fetal weight and wellbeing assessment.²⁰

Failure to end in a successful delivery can be encountered in 15–20% of these interventions and especially in nulliparous women.²¹^,²² Women undergoing IOL require a multitude of additional interventions, including medication, longer hospital stay and monitoring. All contribute to higher levels of mental and physical stress to women, their families and healthcare providers as well as a higher burden on healthcare resources. Additionally, some of these interventions are associated with an increased rate of maternal, fetal and neonatal morbidity and the likelihood of operative intervention.²³ Hence, the ongoing endeavors to identify and evaluate potential predictors of IOL outcome. Several studies have been conducted with the main aim of exploring and/or assessing predictors of a variety of IOL-related maternal and fetal core outcomes.

The Bishop score has been used for assessment prior to IOL to determine the method of induction and the likelihood of its success. Recently it has been reported that Bishop score, as a test, has a poor predictive value, high inter- and intraobserver variability, and therefore, poor reproducibility.²⁴^,²⁵^,²⁶ Hence, ultrasound for cervical assessment has been emphasized as a more objective and accurate tool than digital examination.²⁷ Among the suggested predictors of IOL success are:

Cervical measurement

Assessing CL by transvaginal ultrasound has been shown to perform better than the Bishop score in the prediction of induction-to-delivery interval and IOL outcome.²⁸^,²⁹ It was also reported that the odds of Cesarean section (CS), due to failed IOL, increased by approximately 10% with each 1-mm increase in CL above 20 mm. Moreover, these odds were 75% lower in parous, compared to nulliparous women for the same CL.³⁰ Independent prediction of induction-to-delivery interval and the outcome of IOL within 24 hours were also found possible by ultrasound measurement of CL and posterior cervical angle in a study conducted by Rane et al.³¹ A posterior cervical angle of more than 120° was associated with a positive response to IOL within 24 hours. They also concluded that sonographic parameters were superior to the Bishop score in predicting the outcome of induction.³¹ Furthermore, CL assessment before IOL can be used to identify women who are candidates for prostaglandin administration.³²^,³³ A CL of ≥30 mm or ≥26 mm corresponded to a Bishop score of less than 6 and 4, respectively, defining the need for prostaglandin for cervical ripening.

Identification of fetal head position and station

Fetal spine and occiput position, as predictors of IOL success, have been explored with conflicting results. Pre-induction fetal occiput and spine position determination by ultrasound was not found to correlate with the mode of birth in some studies while others demonstrated fetal head position (Figure 1) to be a significant predictor.³¹^,³⁴^,³⁵^,³⁶ Nonetheless, it is worth noting that there is paucity of data comparing the change in fetal head position during the course of labor in women with intact membranes to women with ruptured membranes. In addition to identifying the position of the baby’s head within the pelvis, it was also possible to identify measurements that could be used objectively to determine the fetal head level in the pelvis. The angle of progression (AoP) is defined as the angle between a line running longitudinally in the midline of the symphysis pubis and a line from the lowermost point of the symphysis pubis tangential to the most leading point of the fetal head (Figure 2).³⁷ Using the AoP to assess fetal head station by transperineal ultrasound (TPU) was found to be a significant independent predictor of outcome and IOL success in nulliparous women.³⁶^,³⁸^,³⁹ In contrast, Pereira et al. found that AoP did not correlate with the outcome of IOL in their study cohort even though it was significantly associated with preinduction CL.⁴⁰ Another parameter that has been explored is the fetal head-to-perineum distance (HPD) (Figure 3), which is measured as the distance between the fetal head and maternal perineum when applying the transducer horizontally transperineally with an upward inclination towards the symphysis pubis (Figure 4).⁴¹^,⁴² HPD was found to successfully predict vaginal delivery after induction of labor, with a predictive value comparable to that of CL assessed by ultrasound and Bishop score.⁴³

Schematic representation (left) and ultrasound image (right), demonstrating transabdominal ultrasound assessment of fetal head position. Dashed lines denote the path of the ultrasound beams.

Schematic representation (left) and ultrasound image (right), demonstrating transperineal ultrasound assessment of angle of progression (AoP). Dashed lines indicate AoP.

Ultrasound image (left) and schematic representation (right), demonstrating transperineal ultrasound assessment of head-to-perineum distance (HPD).

Mannequins showing transducer placement for transperineal ultrasound examination. Vertical placement (left) is used to obtain midsagittal view for measurement of angle of progression (AoP), head-to-symphysis distance (HSD), head-to-perineum distance (HPD) and head direction, while horizontal placement (right) is used to obtain axial view for measurement of HPD, midline angle of rotation (MLA) and subpubic arch angle (SPAA).

Pelvic floor dimensions and function

Levator hiatal dimensions (Figure 5) and IOL outcome have been the focus of several research studies. A correlation between the anteroposterior diameter of the levator hiatus at rest and at maximum Valsalva with the mode of delivery and the duration of the second stage, when assessed before the onset of labor, was reported by independent groups.⁴⁴^,⁴⁵^,⁴⁶^,⁴⁷ Additionally, the behavior of the levator ani muscle on maximal Valsalva maneuver has been studied in pregnant women in whom it was observed that some manifest dysfunctional levator ani muscle contraction, and a consequent reduction in levator hiatal diameter, on pushing, known as levator ani coactivation. Coactivation was demonstrated to be associated with adverse labor outcomes.⁴⁶^,⁴⁷ Interestingly, coaching and ultrasound-aided visual feedback to demonstrate to women how to push down properly was found to be useful in alleviating the dysfunctional muscle contraction.⁴⁸ Therefore, its identification and management prior to IOL is recommended.

Transperineal ultrasound image of midsagittal view (left) and schematic representation of coronal view (right) of levator hiatus, demonstrating anteroposterior diameter (dashed lines).

Prediction models

A number of predictive models and calculators, comprising sonographic and clinical parameters, have been devised and validated for the success of IOL with varying reported results. The clinical predictors most commonly included were maternal age, parity, body mass index, gestational age, Bishop score and estimated fetal weight, while the ultrasound predictors most commonly included were CL and posterior cervical angle, AoP, HPD and fetal occiput position.³⁶^,⁴⁰^,⁴⁹^,⁵⁰^,⁵¹^,⁵²^,⁵³ The search for the ultimate model that enables accurate selection of patients eligible for IOL will continue. The fact remains that these prediction models and calculators serve as guides for the obstetrician’s decision-making and the woman’s informed choice.

PRELABOR ULTRASOUND PELVIMETRY

The use of ultrasound for pelvimetry offers an acceptable, readily available, objective tool without exposing women to unnecessary irradiation. Ultrasound assessment of the maternal subpubic arch angle (SPAA) was found to be a predictor of operative delivery and to correlate with the persistence of the different occiput positions during the process of labor.⁵⁴^,⁵⁵ It was also reported that SPAA correlates with the duration of the second stage.⁵⁶ SPAA is measured by TPU with a horizontal application of the transducer (Figure 4) at the level of the clitoris and tilted upwards until both pubic rami are visible converging into the SPAA which is then measured (Figure 6).⁵⁷ SPAA can also be measured using three-dimensional (3D)-TPU by acquiring a volume of the pelvic outlet.⁵⁸^,⁵⁹

Transperineal ultrasound image (left) and schematic representation (right), demonstrating assessment of subpubic arch angle (SPAA).

Measuring the obstetric conjugate by transabdominal ultrasound has been recently introduced. The transducer is placed at the level of the symphysis vertically to visualize the symphysis pubis and interpubic fibrocartilaginous disc. The distance between the inner edge of the interpubic disc and the sacral promontory, identified as the most prominent segment of the sacral vertebral column, is then measured representing the obstetric conjugate.⁶⁰

ULTRASOUND ASSESSMENT OF PROGRESS OF LABOR

Monitoring of labor progress has classically been done clinically. Labor progress charts were constructed based on clinical assessment of uterine contractions and digital vaginal examination for assessing cervical effacement and dilatation, the fetal presenting part, position and station. However, digital assessment of fetal head position and descent has been proved unreliable with high degrees of interobserver variability. This brought about the need for the introduction of ultrasound in monitoring labor progress offering an available, easily applicable, reproducible objective method of assessment.⁶¹^,⁶²^,⁶³

The use of ultrasound to monitor fetal head descent was proposed as early as 1977 but the methodology was not clinically implementable.⁶⁴ Sherer et al. described a simpler transabdominal approach for diagnosis of fetal head engagement but was not widely practiced due to shadowing from the bony maternal pelvis.⁶⁵ To avoid this predicament, studies that followed focused on TPU (translabial) scanning techniques. TPU parameters for head engagement and descent were more reproducible and widely applicable. AoP, progression distance, head-to-symphysis distance and HPD were all proven to be accurate, reproducible methods for assessing fetal head station and descent.⁶⁶^,⁶⁷^,⁶⁸ Progression distance (PD) is measured as the distance between the infrapubic line (a perpendicular line to the lowermost point of the sagittal midline of the symphysis pubis) to the deepest point of the fetal head.⁶⁸ The head-to-symphysis distance (HSD) is the distance between the lower edge of the pubic symphysis and the nearest point of the fetal skull.⁶⁹ Measuring the distance between the deepest point of the fetal head and the infrapubic line was also suggested to assess the fetal head station as the infrapubic line is 3 cm above the plane of the ischial spines.⁷⁰ For the clinical application of these parameters and how to employ them synergistically for the objective assessment of the progress of labor and aiding management of prolonged labor, HPD and AoP cut-offs of 40 mm and 110° were suggested to be predictive of vaginal birth in 87% and 93%, respectively.⁴²

Assessment of fetal head descent by transabdominal examination has been recently revisited.⁷¹ The reason for this is that the TPU approach, as well as vaginal digital examination, assess the distal part of the fetal head while the transabdominal approach assesses the proximal part. A distinction of which part of the fetal head is being assessed should be considered in cases of fetal skull molding, before assessing fetal head engagement and descent to monitor labor progress or as prerequisites for assisted vaginal birth.⁷² Furthermore, the transabdominal approach tends to be more familiar to obstetricians and more acceptable to laboring women. The suprapubic descent angle (SDA) is now being proposed as a transabdominal ultrasound assessment of fetal head descent and its reliability and reproducibility have been assessed. SDA is the angle between a longitudinal midline through the symphysis pubis and a line from the upper part of the symphysis pubis extending tangentially to the fetal skull (Figure 7).

Ultrasound image (left) and schematic representation (right) demonstrating transabdominal ultrasound assessment of suprapubic angle of descent (SDA). Angle measurement is shown by dashed lines.

Certain ultrasound parameters pertain more to the assessment of the progress of the second stage of labor e.g. the midline rotation angle (MLA) of the occiput, hence the head position and assessing the direction of the fetal head (Figure 8). MLA can be obtained by applying the transducer horizontally on the perineum to display an axial view of the pelvis. It is measured as the angle between the longitudinal fetal head diameter represented by the fetal brain midline structures and the midsagittal anteroposterior diameter of the maternal pelvis.⁷³ Direction of the fetal head in relation to the symphysis pubis and infrapubic line can be visualized by TPU when the fetal head points downwards at higher stations and upwards at lower head stations and more advanced labor stages (Figure 9).⁷⁴ These two parameters are best applied clinically in the active phase of the second stage, when the fetal head is close enough to the perineum to allow for visualization of the brain midline structures and when the change in fetal head direction related to maternal bearing down can be monitored. A narrower angle of progression at the beginning of the second stage in cases of occiput posterior position is associated with a higher risk of persistent occiput posterior position and of Cesarean delivery.⁷⁵ However, serial ultrasound assessments rather than a single one have been proposed to monitor labor progress.⁷⁶ Longitudinal studies and randomized controlled trials are now needed to refine and validate an ultrasound-based partogram.

Transperineal ultrasound assessment of midline angle of rotation (MLA). Angle measurement is shown by dashed lines.

Transperineal ultrasound assessment of fetal head direction. Arrows point to the direction of travel of the fetal head through the birth canal.

ULTRASOUND AND ASSISTED VAGINAL BIRTH

Several intrapartum ultrasound parameters provide helpful tools when an operative vaginal delivery is considered. In such situations, there is a need for timely, confident and evidence-based management decisions. To minimize maternal and neonatal morbidity and, in some cases, mortality, several international bodies and task forces have published guidelines that clearly describe prerequisites to be met before contemplating an assisted vaginal birth.⁷⁷^,⁷⁸^,⁷⁹ Assessing fetal head engagement and descent are fundamental. Likewise, identifying the exact fetal head position is essential for the choice of instrument, its application and the mechanism of a safe birth. Locating the flexion point (3 cm anterior to the occiput) is needed prior to applying the vacuum cup to the head. Similarly, identifying the exact position of the occiput is crucial prior to considering any type of forceps delivery.

Ultrasound assessment of fetal head position was found to be superior to clinical assessment of fetal head position prior to instrumental delivery. In a randomized controlled trial setting, the incidence of incorrect diagnosis was significantly lower in the ultrasound group than in the standard-care group (1.6% vs 20.2%) without delaying delivery.⁸⁰

Reassuring intrapartum ultrasound parameters before operative vaginal delivery include:⁸¹^,⁸²^,⁸³

A well-flexed occiput anterior head position
Midline angle <45°
Head perineum distance <35 mm, in occiput anterior position (or <25 mm regardless of the occiput position)
Angle of progression >145° (>153° on pushing)

Further information on ultrasound evaluation of assisted vaginal delivery is provided in another chapter in this volume (Ultrasound Evaluation of Assisted Vaginal Delivery | Article | GLOWM).

THIRD AND FOURTH STAGES OF LABOR

The third and fourth stages of labor should be less eventful compared to the first two stages, yet significant complications can occur during this period. Ultrasound examination during or after the third stage of labor can be useful in specific cases, rather than as a routine procedure. For example, ultrasound assessment of placental separation might be indicated for a prolonged third stage of labor but should certainly not be performed for every vaginal birth.

Separation and expulsion of the placenta can be visualized by dynamic ultrasound imaging and, accordingly, the third stage has been divided into four phases:⁸⁴

Latent phase: thick placenta-free wall and thin placenta-site wall
Contraction phase: thickening of the placenta-site wall
Detachment phase: separation and detachment of the placenta
Expulsion phase: sliding movement of the placenta

Placental separation has been described as multiphasic in nature commencing from the lower pole of the placenta. Additionally, color Doppler studies suggest that cessation of blood flow between the basal placenta and myometrium is the sonographic hallmark of normal placental separation and that persistent blood flow is suggestive of placenta accreta.⁸⁵ In cases of a retained separated placenta that fails to respond to conservative measures and on resorting to cord traction or manual removal, such maneuvers are best done under ultrasound guidance to guard against acute uterine inversion or incomplete removal.

Ultrasound features of abnormally invasive placenta (AIP) have been described and guidelines published for prenatal (first-, second- and third-trimester) detection.⁸⁶^,⁸⁷^,⁸⁸^,⁸⁹ Nevertheless, there will remain cases that escape prenatal diagnosis. Postpartum diagnosis of AIP by ultrasound has been explored and the loss of the retroplacental hypoechoic zone and thinning of the myometrium beneath the placenta were among the reported findings.⁹⁰^,⁹¹

PUERPERIUM

Scanning the uterus in the normal uncomplicated postpartum period will give different appearances.⁹² The size and shape will vary according to the postpartum phase.

Doppler studies tend to reveal general hypervascularity that is usually insignificant.⁹³

Typically, a thickened endometrial stripe with echogenic tissue in the cavity will be noted. However, these were found not to correlate with complications in asymptomatic patients.⁹⁴ It has been reported that hyperechoic endometrial tissue is seen in 50% and 21% of women 7 days and 2 weeks after delivery, respectively.⁹⁵

Echogenic endometrial tissue is also a common finding in patients complaining of bleeding but was found to have no predictive value for the need for intervention.⁹⁴

Ultrasound has an indisputable role in the diagnosis and management of some postpartum complications. Ultrasound criteria suggestive of retained products of conception (RPOC) include:⁹⁶

Endometrial mass
Heterogeneous fluid in the uterine cavity
Thickened endometrium of more than 10 mm
Vascularity of the endometrial mass as detected by Doppler ultrasound

Nevertheless, the absence of vascularity of the endometrial mass has low negative predictive value as some RPOC may be avascular.⁹⁷ However, increased vascularity of the uterus detected by color Doppler is more prevalent in RPOC when compared to normal postpartum uteri (79% vs 40%).⁹⁶ The presence of an intrauterine mass in asymptomatic women, in the postpartum scan, was found to be predictive of bleeding and need for intervention.⁹⁸ Sonohysterography is conclusive in the diagnosis of RPOC as the saline instilled in the uterine cavity serves as a contrast medium delineating the retained products. The evacuation of RPOC is, indeed, best done guided by ultrasound.⁹⁹^,¹⁰⁰

Ultrasound, transabdominal or transvaginal, is the method of choice for diagnosis of broad ligament hematoma, demonstrating its relationship to the pelvic organs. It provides a quick and readily available diagnostic tool facilitating urgent treatment when needed. Paravaginal hematomas can also be visualized by ultrasound effectively after vaginal delivery.¹⁰¹^,¹⁰² This can be achieved by transabdominal ultrasound, necessitating a full bladder during scanning, or by TPU. There is also a role for ultrasound in the follow up of cases of hematomas that will be managed expectantly and it is now considered as the first-line imaging tool.¹⁰³^,¹⁰⁴ Although uterine rupture is a complication that is suspected clinically, the presence of free fluid in the abdominal cavity, loss of uterine wall continuity, and an empty uterus with a fetus and/or placenta outside the uterine cavity seen on ultrasound remain the hallmarks of diagnosis, especially in cases of epidural analgesia or the rare cases of uterine rupture during pregnancy.¹⁰⁵

ULTRASOUND AND CHILDBIRTH-RELATED PELVIC FLOOR TRAUMA

Endoanal ultrasound has been considered the gold standard for diagnosing obstetric anal sphincter injuries (OASIs). Transvaginal two-dimensional ultrasound was introduced to examine the integrity of the anal sphincter complex.¹⁰⁶ Recent studies recommend the use of 3D-TPU acquired volumes which enables visualization of the whole length of the anal sphincter complex.¹⁰⁷ Strong agreement between endoanal ultrasound and TPU in assessing residual obstetric anal sphincter defects has been established.¹⁰⁸

Levator ani trauma is a known complication of vaginal delivery, occurring more frequently than OASIs. This condition is associated with pelvic floor dysfunction and pelvic organ prolapse. Levator ani avulsion is encountered more among assisted vaginal deliveries, especially forceps deliveries. A systematic review and meta-analysis of levator ani avulsion reported the incidence following Cesarean, spontaneous, vacuum and forceps to be 1%, 15%, 21% and 52%, respectively.¹⁰⁹ The study’s authors also concluded that ultrasound and MRI are comparable tools for levator ani defect diagnosis. Volumetric ultrasound assessment is the mainstay for both static and dynamic levator ani assessments to detect cases of levator ani avulsion (Figure 10) and levator hiatal ballooning.¹¹⁰ This modality has been found to be reproducible and reliable.¹⁰⁹

(a)

(b)

Ultrasound images (left) and schematic representations (right), showing unilateral (a) and bilateral (b) levator ani avulsion.

ROLE OF ULTRASOUND POST CESAREAN SECTION

The key to decreasing the maternal and neonatal morbidity associated with AIP is identifying women at high risk and detecting the condition early. Ultrasound criteria for prenatal detection of AIP have been established as placental lacunae, loss of the hypoechoic space between the placenta and the myometrium, abnormal uterovesical interface, hypervascularity within the placenta and in the subplacental zone and a reduced myometrial thickness (less than 1 mm measured between the echogenic serosa and the retroplacental vessels).⁸⁶^,¹¹¹ The increase in incidence of AIP has been attributed to the increasing Cesarean section (CS) rate.¹¹²^,¹¹³ The presumed etiology is that the number of post-Cesarean decidual scars in the uterus increases the likelihood of a scar pregnancy. The higher likelihood of AIP after elective Cesarean birth was noted.¹¹⁴ This could be explained by the uterine position and integrity of the Cesarean scar rather than by the number of uterine scars. High uterine scar position and the presence of a uterine scar niche were postulated to act synergistically to increase the likelihood of scar/myometrial implantation in a future pregnancy.¹¹⁵ The uterine scar level and its integrity were shown to be related to the stage of labor at the time of Cesarean birth.¹¹⁶

Transvaginal ultrasound and saline contrast sonohysterography are essential for identifying the site and integrity of the CS scar.¹¹⁷^,¹¹⁸^,¹¹⁹^,¹²⁰ Once externally validated, these findings will help stratify women who had delivered by CS into low or high risk for developing AIP which can modify the management in subsequent pregnancies.

Assessment of the scar of a previous CS is also required for proper selection of cases for trial of vaginal birth after Cesarean section (VBAC). Residual myometrial thickness (RMT) and the change in RMT with advancement of pregnancy were found to be predictive and were incorporated into predictive models for the success of VBAC.¹²¹ Ultrasound morphological criteria of the scar have been studied. A scar thickness of 3.5 mm or more, the homogeneity of the scar, scar triangular shape rather than circular or ballooned, qualitatively richer vascular perfusion, and scar volume verified by 3D technique up to 10 cm were found to be attributes of the quality of the scar.¹²² Therefore, ultrasound evaluation of the integrity of the scar can be factored in when counseling for and deciding on the mode of delivery in women who had prior CS delivery.

The incidence of Cesarean section scar pregnancy (CSP) has been on the rise due to the increased incidence of CS and due to the improvements in diagnostic techniques allowing for early diagnosis. Transvaginal ultrasound is the hallmark for diagnosis. The earlier the diagnosis the easier it is (Figure 11). With advancing gestation, the precise location of the sac in relation to the CS scar becomes more difficult to determine.¹²³

Transvaginal ultrasound image at 5 weeks’ gestation, showing Cesarean scar pregnancy.

With sinister complications, such as AIP, and possible uterine rupture, the importance of early detection and management of CSP cannot be overemphasized. Guidelines for ultrasound-based criteria issued by the RCOG include: a gestational sac or solid mass of trophoblast embedded at the site of the Cesarean scar; empty endocervical canal; thin or absent layer of myometrium between gestational sac and anterior uterine wall/bladder; evidence of prominent vasculature.¹²⁴ Diagnosing CSP using the location of the gestational sac in relation the uterine midpoint was also suggested.¹²⁵ Another study added that, on a sagittal view of the uterus running through the amniotic sac, a discontinuity in the anterior wall of the uterus should be identified. The importance of the differential diagnosis of an avascular gestational sac in cases of miscarriage was highlighted.¹²⁶

In cases of CS scar endometriosis, the primary diagnosis is achieved clinically. Typically, the patient reports an abdominal wall mass related to the incision site of a CS, that appears and is painful with menstruation only. Ultrasound is conclusive as to the nature of the mass. Unlike adnexal endometriosis, when implanted on scars, they appear as solid heterogeneous masses with echogenic residues or strands. The echogenic pattern depends on the hemorrhagic and fibrous content. The appearance varies according to the amount of intralesional menstrual bleeding. They often show irregular spiculated margins. Large-sized CS scar endometriomas, more than 3 cm, are more cystic with less defined borders.¹²⁷^,¹²⁸

CONCLUSION

It is evident that most of the recent advances in obstetric care have been related to the application of ultrasound in clinical practice, including in the context of labor and delivery. Ultrasound is an indispensable tool in contemporary obstetrics. Its use has been generalized to the point that we have transitioned from ‘obstetric ultrasound’ to ‘ultrasonographic obstetrics’.¹²⁹ Therefore, it is recommended that the use of ultrasound scanning in the labor ward be incorporated within unit protocols with widescale accessibility to scanning machines. Furthermore, it should be considered as a core component of training in obstetrics and midwifery.

PRACTICE RECOMMENDATIONS

Ultrasound is considered a complementary tool for management of labor and should be available for all birth attendants.
Ultrasound assessment of fetal viability and wellbeing, presentation and position, placental location and estimation of fetal weight should be performed in women prior to labor.
In cases of antepartum hemorrhage, ultrasound is carried out for diagnosis of placental location and possibly abruption.
In cases of threatened preterm labor, for which digital examination is not recommended, transvaginal ultrasound can be safely employed to assess the cervix by measuring the cervical length, funneling and cervical index.
Ultrasound is recommended in pregnant women scheduled for induction of labor for assessment of eligibility and prediction of success.
- For assessment of eligibility: estimation of fetal weight and wellbeing, confirmation of gestational age, fetal presentation, and placental and umbilical cord position.
- For predicting success:
  - Cervical assessment by transvaginal ultrasound to measure cervical length and posterior cervical angle.
  - Estimation of fetal occiput position by transabdominal ultrasound.
  - Estimation of fetal head station using transperineal ultrasound by measuring the angle of progression or head-to-perineum distance.
  - Assessing levator hiatal dimensions, levator ani function and excluding levator coactivation.
  - For incorporating ultrasound parameters with clinical parameters in predictive models to assess likelihood of success.
  - Ultrasound can be used for prelabor assessment of the shape of the pelvis by measuring the subpubic arch angle and the obstetric conjugate.
Ultrasound can be employed to monitor progress of labor and aid in the management of complicated cases by a variety of methods:
- Estimation of fetal head descent using transperineal ultrasound by measuring the angle of progression, head-to-perineum distance, head-to-symphysis distance or progression distance.
- Estimation of fetal head descent using transabdominal ultrasound by measuring the suprapubic distance.
- Estimation of fetal head direction and midline angle of rotation in second stage of labor.
Ultrasound can play a decisive role prior to assisted vaginal delivery:
- Assessing fetal head engagement by suprapubic descent angle and fetal head descent by angle of progression, head-to-perineum distance or progression distance.
- Identifying the exact fetal head position and midline angle of rotation is essential for the choice of instrument, its application and the mechanism of a safe birth. Locating the flexion point (3 cm anterior to the occiput) is needed prior to applying the vacuum cup to the head. Similarly, identifying the precise position of the occiput is crucial prior to considering any type of forceps delivery.
Ultrasound assessment of placental separation might be indicated for a prolonged third stage of labor but should not be done routinely.
Ultrasound can identify signs of an abnormally invasive placenta.
Puerperal uteri can be followed up by ultrasound for size and shape through the different involution phases. Ultrasound has an indisputable role in the diagnosis and management of some postpartum complications such as identification of retained placental cotelydons.
Postdelivery maternal-birth-tract injuries, such as uterine rupture, and broad-ligament, paravaginal and vulval hematomas, are all easily and most precisely diagnosed by ultrasound.
Childbirth-related pelvic floor trauma in the form of obstetric anal sphincter injury or levator ani avulsion can be easily and accurately assessed using two- and three-dimensional transperineal ultrasound with an efficacy comparable to those of endoanal ultrasound and MRI.
In a pregnancy after a prior Cesarean delivery, the following are recommended:
- Early ultrasound assessment to exclude Cesarean scar pregnancy.
- Accurate location of the placenta and identification of ultrasound signs of abnormally invasive placenta.
- Measurement of lower uterine segment myometrial thickness before trial of labor.

CONFLICTS OF INTEREST

The author(s) of this chapter declare that they have no interests that conflict with the contents of the chapter.

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