Prediction of persistent occiput posterior position by sonographic assessment of fetal head attitude at start of second stage of labor: prospective study

To evaluate the relationship between the attitude of the fetal head quantified by means of the chin‐to‐chest angle (CCA) in fetuses in occiput posterior (OP) position at the beginning of the second stage of labor, and persistent OP position at birth.

#R.R.Z. and A.D.'A.are joint first authors.
Accepted : 11 August 2023   position, station by means of angle of progression and head-to-perineum distance, and attitude by means of CCA were assessed using transabdominal or transperineal ultrasound.The primary outcome was persistent OP position at birth.

INTRODUCTION
Occiput posterior (OP) position of the fetal head is encountered in around 20-30% of fetuses at full cervical dilation 1 .Among them, most will rotate spontaneously into an occiput anterior (OA) position and only 5-10% will deliver in a persistent OP position 2,3 .The latter condition is a major contributor to labor dystocia, leading to an increased frequency of operative delivery and adverse maternal and neonatal outcomes [4][5][6][7] .
In an attempt to improve vaginal delivery rates and maternal and neonatal outcomes, researchers have advocated the prophylactic use of manual rotation of the fetal head.However, recent randomized clinical trials have failed to show a clear benefit of this intervention in improving labor outcome [8][9][10][11] .One explanation might be the lack of identification of fetuses at high risk of persistent OP position and labor dystocia, which might benefit from this intervention 12 .Another explanation is related to the intervention per se, which might be more difficult to perform in deflexed OP fetuses, hence reducing the chance of success 13 .
To date, there is no clear explanation as to why some fetuses rotate spontaneously and others persist in an OP position.Several risk factors for persistent OP position, including maternal and neonatal characteristics 3,14 and intrapartum sonographic parameters, have been described 15,16 .Recently, a retrospective study conducted on OP fetuses reported an association between clinically assessed head attitude and spontaneous rotation into an OA position at birth 17 .However, vaginal digital examination is unreliable in the assessment of fetal head attitude and its accuracy is affected by factors such as caput succedaneum and molding of the fetal head.We have recently proposed the quantitative sonographic assessment of the fetal head attitude in OP fetuses by means of the chin-to-chest angle (CCA), demonstrating an association between head deflection and Cesarean delivery due to labor arrest 18 .
In this study, we aimed to evaluate the relationship between the flexion of the fetal head, quantified by means of the CCA, in fetuses in OP position at the beginning of the second stage of labor and persistent OP position at birth.

METHODS
This was a single-center, prospective observational study conducted at the University Hospital of Parma, Parma, Italy.A non-consecutive series of laboring women with a fetus in OP position at full cervical dilation, and prior to the beginning of the pushing efforts, were deemed eligible.After providing written consent, patients were recruited into the study.The inclusion criteria were singleton pregnancy with non-anomalous fetus and gestational age at or beyond 37 weeks.Cases of obstetric intervention due purely to suspected intrapartum fetal compromise were excluded from data analysis.The study was approved by the local ethics committee (270/2018/OSS/AOUPR).
As per local protocol, and using a Samsung HM70 (Samsung Medison, Seoul, South Korea) ultrasound machine, all patients with suspected OP position at clinical examination performed at full cervical dilatation underwent transabdominal ultrasound by the senior obstetrician responsible for the patient care to confirm the clinical findings (i.e. the clinical scan).The fetal head position was ascertained by means of transabdominal ultrasound and described as a clock face with 12-h divisions, according to the International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) guidelines 19 , and OP position was assigned when the fetal occiput was located > 03.30 h and < 08.30 h.Following sonographic confirmation of the OP position, a second obstetric member of the research team, but not involved in the clinical management, performed additional transabdominal and transperineal sonographic measurements (i.e. the research scan) to evaluate the fetal head attitude and station.The fetal head attitude was assessed quantitatively by means of the CCA, and the fetal head station by means of the angle of progression (AoP) and head-to-perineum distance (HPD).First, the measurement of the CCA was performed transabdominally.The CCA was determined, as previously described 18 , by the internal angle formed by two lines: (1) a first line running tangential to the longest axis of the fetal sternum and (2) a second line tangential to the skin covering the inferior boundary of the oral cavity up to the fetal chin (Figure 1).The other sonographic parameters, such as the HPD 20 and the AoP 21 were measured transperineally, as previously described.The senior obstetrician in charge of the patient care was blinded to the findings of the research scan.
The primary outcome of the study was persistent OP position at birth.Maternal, delivery and neonatal data were retrieved from the medical records.Maternal data included maternal weight and body mass index at presentation, maternal height, age, ethnicity, parity, gestational age at delivery, induction and augmentation of labor and epidural use.Delivery characteristics included fetal head position, fetal spine position, AoP, HPD, CCA, mode of delivery, occiput position at delivery, perineal tear and blood loss.Neonatal data included birth weight, 5-min Apgar score, and umbilical artery pH and base excess.Data were recorded and stored in a Microsoft Excel (Microsoft Corp., Redmond, WA, USA) secured

Statistical analysis
We calculated the sample size as follows: the incidence of persistent OP position among fetuses that started the second stage of labor in an OP position is approximately 30%.Under the assumption of 80% power and an alpha error of 0.05, we calculated we would need 76 patients to demonstrate that a wide CCA might result in a 50% increase (from 30% to 45%) in the incidence of the primary outcome (persistent OP position) in the study population 22 .
Statistical analysis was performed using Statistical Package for Social Sciences (SPSS) version 22 (IBM Corp., Armonk, NY, USA).The Kolmogorov-Smirnov test was used to assess the normality of the distribution of the data.Statistical analysis was performed using the chi-square test for categorical variables and the Student's t-test or the Mann-Whitney U-test as appropriate for continuous variables, and the results are presented as n (%), mean ± SD or median (range).Multivariable logistic regression analysis was used to control for potential confounding variables.The continuous variables included in the logistic regression model were converted to Z-scores.The prediction of primary outcome was determined by receiver-operating-characteristics-curve analysis.P < 0.05 was considered as statistically significant.This study was conducted following the STROBE guidelines 23 .

RESULTS
Between January and July 2022, 81 women were eligible for the study.Among these, five were excluded due to obstetric intervention for suspected intrapartum fetal compromise, leaving 76 women for data analysis (Figure 2).Among the included cases, vaginal delivery occurred in 56 (73.7%), and operative vaginal delivery of the fetal head during the second stage of labor or persistent OP position, are summarized in Table 1.There were 48 (63.2%) fetuses with spontaneous rotation of the fetal head into OA; all of them had a spontaneous vaginal delivery.Among the 28 (36.8%)OP fetuses not rotating to an OA position at delivery, eight (28.6%) had a spontaneous vaginal delivery, while operative vaginal delivery and Cesarean delivery were performed in 11 (39.3%) and nine (32.1%) cases respectively.Persistent OP position was associated with a higher frequency of nulliparity (24/28 (85.7%) vs 27/48 (56.3%);P = 0.008), augmentation of labor (20/28 (71.4%) vs 23/48 (47.9%);P = 0.04), a wider CCA (54.9 • ± 26.2 • vs 39.8 • ± 21.0 • ; P = 0.007) and longer duration of labor (545 ± 227 min vs 415 ± 164 min; P = 0.008) compared to cases with spontaneous rotation of the fetal occiput into an OA position.In this study, we did not observe an association between the AoP (127.6 • ± 15.2 • vs 121.7 • ± 16.3 • ; P = 0.16) and HPD (39.0 ± 14.4 mm vs 37.1 ± 13.4 mm; P = 0.60) in fetuses with a persistent OP position at birth compared to those which rotated spontaneously into an OA position.
We found 29/76 (38.2%) fetuses with a CCA < 36.5 • , of which 24 (82.8%)rotated spontaneously and had a spontaneous vaginal delivery in OA position, two (6.9%) did not rotate spontaneously but had a spontaneous vaginal delivery in OP position, one (3.4%)was delivered by operative vaginal delivery and two (6.9%) by Cesarean section.Furthermore, there were 47/76 (61.8%) fetuses with a CCA ≥ 36.5 • , of those 24 (51.1%)spontaneously rotated and had a spontaneous vaginal delivery in OA position, six (12.8%) did not rotate spontaneously but had a spontaneous vaginal delivery in OP position, 10 (21.3%) underwent operative vaginal delivery and seven (14.9%) Cesarean section (Figure 3).

DISCUSSION
This study demonstrates that in OP fetuses at the beginning of the second stage of labor, the degree of flexion of the fetal head, measured by means of the CCA, and parity were the only predictors for persistent OP position at birth.
Persistent OP position is associated with an increased frequency of operative delivery and adverse maternal and neonatal outcomes [4][5][6][7] .Previous studies using ultrasound have attempted to identify predictors of spontaneous rotation of the fetal head from OP to OA position.In a prospective cohort study of 84 women enrolled during the second stage of labor, Blasi et al. 16 found an association between a posterior spine position and persistent OP position.In this study, we could not replicate these results, as we did not find a significant difference in the fetal spine position between our groups (Table 1).More recently, Brunelli et al. 15 investigated the relationship between the fetal head station and persistent OP, showing a relationship between a narrow AoP at the beginning of the second stage of labor and a higher incidence of persistent OP position.However, in their study, the authors did not evaluate the degree of flexion of the fetal head.This is important because a deflexed fetal head is associated with higher station (i.e.narrower AoP) 18 and downward direction of descent 24,25 .As such, it is likely that the fetal head attitude and not the fetal head station per se is associated with spontaneous rotation of the fetal head into an OA position at birth, as shown in this study.
Fetal head attitude in OP fetuses is a possible determinant of spontaneous rotation to an OA position 26 .To our knowledge, only one study has so far evaluated the association between the degree of flexion of the fetal head and the chance of spontaneous rotation into the OA position.Othenin-Girard et al. 17 conducted a retrospective study on 439 fetuses in OP position and reported that a good degree of flexion of the fetal head, evaluated clinically, was the only significant predictor of spontaneous rotation into OA position at delivery.However, one major limitation of their study is that the clinical assessment of the fetal attitude might be challenging, even for senior obstetricians.
Our research group has recently investigated the role of the fetal attitude in the context of protracted active phase of labor 18 , demonstrating an increased frequency of Cesarean delivery due to labor arrest in more deflexed OP fetuses, defined as those with a CCA on or above 33 • .This figure is consistent with the 36.5 • cut-off optimally predicting persistent OP position, which we found in the present study.This relationship is plausible, as deflexed fetal heads present larger diameters and a tighter spatial relationship with the maternal birth canal, which might interfere with spontaneous fetal head rotation 18,27 .Of note, unlike clinical examination, measurement of the CCA can be considered an objective and reliable method to assess quantitatively the fetal head attitude.Unpublished data from our group suggest very good intra-and interoperator reproducibility of the measurement of the CCA for experienced and inexperienced operators.
Large retrospective studies have reported association between maternal demographics, such as parity, and persistent OP position 3,14,28 .This study showed an independent association between parity and persistent OP position.Importantly, this study did not evaluate the maternal pelvimetry, which may impact on the incidence of persistent OP position 3,29 and the mode of delivery [30][31][32][33] .Indeed, a narrow anterior compartment of the maternal pelvis has been associated with a higher incidence of persistent OP positions, which may explain why some fetuses with a flexed head are delivered vaginally in persistent OP position 3,29 .
Management of fetuses in OP position during the second stage of labor is challenging.Expectant management seems to be the gold standard, as available evidence suggests that the majority of fetuses in OP position rotate spontaneously into OA position by the end of labor 1 .Several attempts have been made to identify maternal positions aimed at reducing the rate of persistent OP position at delivery.Nevertheless, studies have reported that changing maternal position did not result in a reduction of the incidence of persistent OP position [34][35][36] .Over the last decade, several researchers have proposed the prophylactic use of manual rotation of the fetal head to reduce the incidence of persistent OP position and improve maternal and neonatal outcomes [4][5][6]37 . Howver, recent randomized clinical trials have failed to show any clear benefit of the maneuver when performed in the context of an unselected population of OP fetuses [8][9][10][11] .Further studies should clarify if manual rotation of the fetal head may perform differently in patients with additional risk factors, such as those with OP position and deflexed attitude, and those with labor dystocia 12,38 .
Our results show that CCA has a good sensitivity and negative likelihood ratio for the identification of persistent OP position at birth.A fetus in OP position with a CCA below the threshold (36.5 • ) might benefit from expectant management, given that these fetuses are most likely to be delivered in OA position without any intervention.Contrarily, among fetuses in OP position with CCA values above the threshold, manual rotation of the fetal head can be considered, given that 50% of these cases will remain OP at birth.
The main strength of this study is represented by its original design and prospective enrolment of patients with fetuses in OP position at the start of the second stage of labor.Additionally, operators were blinded to the ultrasound measurements of the fetal attitude to decrease the probability of selection bias.
Conversely, the small sample size can be seen as a limitation of this study.However, according to our power sample-size calculation, the number was large enough to detect significant changes in the incidence of persistent OP position at delivery.Therefore, our results should be interpreted with caution and larger studies are required to confirm our results.
In conclusion, our data show that the sonographic fetal attitude measured by means of the CCA might help in the identification of fetuses at risk of persistent OP position within an unselected population of women with OP fetuses at the beginning of the second stage of labor.Further studies should investigate if the CCA might select for patients who may benefit from manual rotation of the fetal head.

Figure 1
Figure 1 Grayscale ultrasound images showing measurement of fetal head attitude by means of chin-to-chest angle in occiput posterior fetuses with (a) well-flexed head (angle, 29.13 • ) and (b) deflexed head (angle, 112.84 • ).A, line running tangential to longest axis of fetal sternum; B, line tangential to skin covering inferior boundary of oral cavity up to fetal chin.

Figure 2
Figure 2 STROBE flowchart summarizing inclusion in study of women with singleton pregnancy at or beyond 37 + 0 weeks of gestation, with fetus in occiput posterior (OP) position at beginning of second stage of labor.Persistent OP position includes fetuses delivered vaginally in OP position and fetuses delivered by obstetric intervention due to labor dystocia during second stage of labor.OA, occiput anterior.

igure 3
Mode of delivery in 76 women with occiput posterior fetus at beginning of second stage of labor, according to chin-tochest angle (CCA) threshold of 36.5 • ., Cesarean section; , operative vaginal delivery; , vaginal delivery in occiput anterior position; , vaginal delivery in occiput posterior position.

Table 2
Logistic regression analysis for association of baseline and intrapartum characteristics in prediction of persistent occiput posterior (OP) position in pregnant women with fetus in OP position at start of second stage of labor

Table 1
Baseline characteristics and perinatal outcome of 76 patients, according to spontaneous rotation of fetal head during second stage of labor or persistent occiput posterior (OP) position Data are given as mean ± SD, n (%) or median (range).*Includes fetuses delivered vaginally in OP position or by obstetric intervention due to labor dystocia during second stage of labor.†At presentation.BMI, body mass index; GA, gestational age.