Sonographic evaluation in the second stage of labor to improve the assessment of labor progress and its outcome

Every pregnant woman in early spontaneous labor or due to undergo induction of labor wishes to know if she will deliver vaginally, or if she will require an operative delivery. Will the energy, pain and effect of labor on the pelvic floor be worth a vaginal delivery? What are the consequences of a failed attempt at a vaginal delivery? It would be ideal if, before the onset of labor or in its early stages, we could predict accurately who will deliver vaginally spontaneously without complications, and who will require an operative delivery (abdominal or assisted). This information would reduce morbidity, improve safety, optimize utilization of resources and improve satisfaction of women with the labor and/or delivery process. The results of a prospective observational study of 202 nulliparous women suggested that mode of delivery could be predicted accurately before the onset of labor in up to 87% of cases, using a combination of clinical and historical factors as well as ultrasound findings1. Sonographic examination has the potential to improve this predictive accuracy. Such an improvement is highly desirable. Until the development of the partogram, in the middle of the twentieth century, evaluation of labor progress was subjective and it was often monitored according to its duration. The partogram was a crucial advance which allowed objective assessment of the progress of labor by plotting cervical dilatation and fetal station as a function of time2-15 (Figure 1), and allowed labor disorders to be defined and classified (prolonged latent phase, protracted labor and arrest disorders). The pelvic division of labor, which encompasses the phase of parturition during which the fetus descends through the birth canal, is defined by changes in station of the presenting part16. It is noteworthy that descent begins before full dilation (Figure 1). The management of protracted descent (nulliparous women, ≤ 1 cm/h; multiparous women, ≤2 cm/h), arrest of descent (no change in station for ≥ 1 h) or failure to descend (no change in the deceleration phase or the second stage)13 is one of the most challenging problems of obstetrics. Indeed, the skill and judgment required to decide when and how to intervene have been defining factors in the art of obstetrics. Partogram developed by Emanuel Friedman which describes the relationship between time and two variables: cervical dilatation and station of the presenting part. Reproduced from: Friedman EA. Dysfunctional labor. In Management of Labor, Cohen WR, Friedman EA (eds). University Park Press: Baltimore, 1983; 11–2415. Operative vaginal deliveries were introduced to deal with obstructed labor. The feasibility of an abdominal delivery and its improved technique and safety, coupled with a concern for fetal and neonatal welfare have changed the components of the obstetric equation to opt for a particular mode of delivery (operative vaginal versus abdominal delivery). Ultrasound has earned its role in obstetrics through the assessment of gestational age and fetal growth, detection of congenital anomalies and assessment of fetal well-being and cervical length. In this Editorial, we reaffirm the notion that ultrasound is superior to digital vaginal examination in determining fetal position prior to operative delivery17-26, and explore the concept that it brings objectivity to the assessment of the location of the presenting part (station), and may contribute substantially to the clinical management of the second stage of labor. This issue of the Journal contains several original contributions which support this view. A pioneering study by Barbera et al.27 (from Professor John Hobbins' group) reports the use of transperineal/translabial ultrasound28, describing that the angle between the symphysis pubis and the leading part of the fetal skull (‘angle of progression’) could predict which patients would deliver vaginally and which were at risk of requiring an abdominal delivery. This concept was originally described on DVD in 2003 (under the sponsorship of the American College of Obstetricians and Gynecologists)29 and is now published in this issue of the Journal27. An angle of progression of ≥ 120° (Figure 2) was associated in all cases with a spontaneous vaginal delivery. The angle was also associated with the duration of the examination-to-delivery interval. The intra- and interobserver variability of measuring the angle of progression were low, and there was a good correlation between clinical assessment of progress and the angle of progression. This concept has been studied independently by Professor Karim Kalache at the Charité University Hospital in Berlin, in the Department of Professor Joachim Dudenhausen30. Among 26 cases with an occiput anterior fetal position, an angle of progression of ≥ 120° was associated with a probability of successful vaginal delivery of 90% (derived from logistic regression). Collectively, the evidence suggests that the simple determination of the angle of progression has important prognostic value. Angle of progression. This is the angle formed between a line placed through the midline of the symphysis pubis, and a second line (dashed) drawn from the inferior apex of the symphysis tangentially to the fetal skull. In this figure, the angle of progression is > 120°. The original figure was provided by Professor Karim Kalache (Berlin, Germany) and has been slightly modified. Engagement of the fetal head is a major prognostic factor for the outcome of labor31-33, but its diagnosis by clinical means has many shortcomings; ultrasound has therefore been used to improve diagnostic accuracy. Engagement is said to have occurred when the widest diameter of the fetal presenting part has passed beyond the plane of the maternal pelvic inlet34. In a cephalic presentation, usually the head is engaged when the leading part of the skull is at or below the level of the maternal ischial spines. The diagnosis of engagement can be made clinically by transvaginal digital examination, or by transabdominal examination (using the fourth Leopold maneuver and finding that two-fifths of the fetal head is palpable)35. Olivier Dupuis et al.36 in Lyon, France, made a seminal contribution by rigorously assessing the accuracy of station diagnosis with digital examination. This was accomplished using a birth simulator to systematically examine the reliability of digital examination by residents (average experience, 2 years) and attending physicians (average experience, 9 years) to determine engagement and fetal station. A fetal head mannequin was placed in 11 possible fetal stations in a random order (from − 5 to + 5) (Figure 3). The error in the diagnosis of station was assessed by calculating the difference between the true station (established by a sensor in the birth simulator) and that determined by the operator using digital examination. Numerical errors occurred in 50–88% of cases in the resident group, and in 36–80% of cases in the attending physician group. The mean ‘group’ error for residents was 30% (95% CI, 25–35%), and for attending physicians was 34% (95% CI, 27–41%). The mean error rate in the diagnosis of engagement was 12%, and this misdiagnosis was distributed equally between false positives and false negatives. Undiagnosed ‘high’ stations accounted for 22.4% of the errors made by residents, and 15.95% of those made by attending physicians. Alarmingly, the misdiagnosis of a station as representing a mid-pelvic station rather than a true high-pelvic station accounted for 87.5% and 66.8% of the errors made by residents and attending physicians, respectively. This study provides evidence that transvaginal digital examination of fetal head station is not reliable, even though it is standard practice to use it to determine labor progress in labor and delivery units around the world. Yet, station is the determining factor for whether an operative vaginal delivery should be performed. High pelvic deliveries (when the head is not engaged) have been abandoned because they are unsafe for mother and fetus. In addition, the risks associated with a mid-pelvic (station above + 2 but head engaged), a low and an outlet operative vaginal delivery are different36 (see Table 1 for definitions34). Relationship between the bony pelvis and the presenting part to define fetal station. It is generally held that when the largest diameter of the fetal head is at the level of the pelvic inlet, the leading part of the head will be at the level of the ischial spines (Sp.) (Station 0). The plus and minus numbers represent centimeters below and above the ischial spines, respectively. Reproduced from: Greenhill JP, Friedman EA. Biological Principles and Modern Practice of Obstetrics. WB Saunders: Philadelphia, 197440. Professor Hans Peter Dietz and his team from Sydney, Australia, proposed a simple method to evaluate the station of the fetal head using translabial ultrasound after studying prospectively 140 nulliparous pregnant women who were not in labor37. A vertical line was drawn from the edge of the pubic symphysis, and a line perpendicular to it was drawn to the leading edge of the fetal head. The minimal distance between these two landmarks is now referred to as the ‘head progression distance’ (Figure 4). Dietz et al. provided evidence that this parameter correlated with the station of the fetal head by clinical examination (by both abdominal palpation and vaginal digital examination), and that this method was highly reproducible. Even though the technique was originally described for the assessment of engagement, it may be valuable in the assessment of progress in the second stage of labor37. The lower the presenting part, the greater the head progression distance. Head progression distance as proposed by Dietz et al.37 to assess the degree of descent of the presenting part. A line through the inferoposterior symphyseal margin (parallel to the main transducer axis) is the reference line. The minimal distance between this line and the leading edge of the presenting part is measured (dashed line marked by calipers) and represents the head progression distance. Two other studies have compared methods for the diagnosis of engagement in women in labor. The first compared two clinical methods (transvaginal digital examination versus transabdominal palpation)38. The second compared transvaginal digital examination with transabdominal ultrasound39. Both studies lacked a gold standard. The fundamental premise which has been used in the labeling of stations is that the birth canal is straight (Figure 3). Yet, this is not the case: while the first part of the birth canal is indeed straight, the second has a curvilinear trajectory40 (Figure 5). Barbera et al.41 report in this issue of the Journal a study in which computed tomography of the pelvis was performed in 70 non-pregnant women. Coordinates were developed to identify specific pelvic landmarks, such as the upper and lower borders of the symphysis pubis, as well as the right and left ischial spines. This information was used to generate a geometric model of the birth canal which allowed calculation of a set of theoretical angles for each station of the fetal head when performing transperineal ultrasound. An angle of 99° corresponded with engagement (Station 0), and the various fetal stations correlated with specific angles. An important finding of this study was that there was a poor correlation between station assessed by digital examination and station assigned by these angles. The discrepancy was greater as the fetal head descended deeper into the birth canal41. An open question is whether the lack of correlation is due to the proven inaccuracy of digital examination. Clearly, a gold standard is required to resolve this issue. A diagram of the birth canal demonstrating that it has a curved trajectory. The presenting part descends in a straight line, and then curves as it approaches the outlet. Reproduced from: Greenhill JP, Friedman EA. Biological Principles and Modern Practice of Obstetrics. WB Saunders: Philadelphia, 197440. A collaborative study between the Charité Hospital of the University of Berlin and the Heinrich-Heine-University in Dusseldorf, Germany (Dr Boris Tutschek) yielded a series of interesting observations with the potential to improve the assessment of progress in the second stage of labor. Professor Wolfgang Henrich et al.42 focused on the study of progress in the second stage of labor by using a transducer translabially to define maternal and fetal landmarks. The main parameters were: 1) the infrapubic line; 2) the widest fetal head diameter; and 3) the head direction (with respect to the long axis of the symphysis pubis) (Figure 6). Head direction was defined by the angle between the infrapubic line and another line drawn perpendicular to the widest diameter of the fetal head. Using this technique, three types of head direction were determined: head down, horizontal and head up (Figure 6). Henrich et al.42 also reported a computed tomographic three-dimensional (3D) reconstruction of the normal female pelvis. This demonstrated that a line parallel to the infrapubic line and 3 cm caudal to it identified the plane of the ischial spines (0 station). This implies that if the widest fetal head diameter is above the infrapubic line, it also has to be far above the mid-pelvis (0 station). Henrich et al. found that when the widest diameter of the fetal head crossed the infrapubic line (thus approaching or crossing the interspinous plane), and especially if the head was up, an operative delivery had a favorable outcome. Poor prognostic signs for vaginal delivery were a lack of descent or lack of passage of the fetal head below the infrapubic line, and a downward or horizontal direction of the head. However, a potential limitation of this approach is that the posterior skull table is not always well-visualized and, therefore, there is some subjectivity in the determination of the widest fetal head diameter. Head direction as described by Henrich et al.42. An infrapubic line (solid) is drawn perpendicular to the long axis of the symphysis pubis. The dotted line represents the widest fetal head diameter. The head direction is the angle formed between the infrapubic line and another line (dashed arrow) drawn perpendicular to the widest diameter of the fetal head. ‘Head up’ is when this latter line points ventrally at an angle of ≥ 30°; head down is when this angle is < 0°; all other angles are considered ‘horizontal’. Dr Tullio Ghi et al., from the University of Bologna, Italy, report in this issue of the Journal a unique study in which a group of patients who reached full dilatation underwent serial translabial sonograms and digital examinations at 15–30-min intervals until delivery43. The parameters evaluated were the head direction as described by Henrich et al.42 (downward, horizontal or upward), as well as the degree of rotation of the fetal head. The latter was accomplished by placing the transducer transversely below the symphysis pubis to visualize the midline of the fetal head and classify the degree of rotation as < 45° or ≥ 45°. The major findings of this study were that: 1) there was a strong relationship between the ‘direction of the fetal head’, as determined by translabial ultrasound, and the station, as determined clinically (Table 2); 2) the orientation of the midline of the fetal head with respect to the anteroposterior diameter of the pelvis (referred to by the authors as ‘the degree of rotation of the fetal head’) was also associated strongly with the station of the fetal head; 3) importantly, the combined findings of an upward direction of the fetal head and a rotation of < 45° were associated in 95% of cases with a + 3 station of the presenting part43. Ghi et al.43 propose an approach for management of the second stage of labor based on ultrasound. Patients with a downward direction of the fetal head would be eligible for an abdominal delivery because the presenting part is above the mid-pelvis. Conversely, those with an upward direction of the fetal head and a degree of rotation of < 45° could be delivered vaginally, either spontaneously or operatively. The management of patients in whom the fetal head is in the horizontal position remains uncertain. The optimal method for assessment of progress when the fetal head is in occiput posterior position is a challenge because of the difficulties in interpreting the direction of the fetal head. Indeed, even when the fetal head was close to the outlet, it tended to remain horizontal, and the angle of rotation could be < 45° when the head was still high at the pelvic inlet. Most studies thus far have relied on two-dimensional sonography. 3D sonography will add additional information44 because the simultaneous visualization of an orthogonal plane improves the reliability that the infrapubic line has been obtained in the correct plane and facilitates acquisition and analysis of the information. A recent development is the availability of software and hardware designed specifically to objectively assess progress in the second stage of labor based on 3D sono- graphy. Studies are required to determine the optimal parameters for monitoring labor progress, diagnosing labor disorders and predicting the likelihood of success of an operative vaginal delivery and neonatal outcome. One of the most difficult dilemmas in clinical obstetrics is that presented by a nulliparous woman at term who had a protracted active phase of labor, received an epidural, and now has a prolonged second stage of labor. The head appears to be low, but there is always uncertainty as to whether this reflects caput. Should this situation be handled with an operative vaginal delivery (forceps or vacuum) or a Cesarean section? An unsuccessful operative vaginal delivery could have serious consequences to both the mother and the fetus. On the other hand, a Cesarean delivery when the head is deep in the birth canal may also lead to serious complications, such as lacerations of the uterus, vagina and bladder, and even fetal injuries caused during the extraction procedure45-56. Ultrasound can play a role by: 1) identifying precisely the fetal position; 2) diagnosing whether caput is present and its extent; 3) assessing the fetal station; and 4), when ultrasound has been used serially, documenting the degree of progress that has been made during the second stage. Presently, obstetricians are required to document the station and the position of the fetal head, as well as the rationale for his/her decision. The accumulating body of literature suggests that ultrasound can play an important role in the objective documentation of landmarks and parameters which are valuable in managing this complex obstetric challenge. This work was supported by the Intramural Research Program of the Eunice Kennedy Shriver National Institute of Child Health and Human Development, NIH, DHHS.