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Keywords:

  • pubic arch angle;
  • second stage of labor;
  • transperineal ultrasound

ABSTRACT

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. METHODS
  5. RESULTS
  6. DISCUSSION
  7. REFERENCES

Objective

To evaluate the clinical significance of the pubic arch angle (PAA) measured by transperineal ultrasound during prolonged second stage of labor.

Methods

We evaluated prospectively 62 women ≥ 37 weeks of gestation with failure to progress in the second stage of labor. Transperineal ultrasound (transverse plane) was used to measure the pubic arch angle. Correlations with fetomaternal characteristics, mode of delivery and perinatal outcome were evaluated.

Results

The mean PAA was 101.1° (± 13.1°; range, 80°–135°). We found a negative correlation with maternal age. Patients with an occipitotransverse fetal position had a significantly smaller angle compared with those with occipitoanterior positions (94.3° ± 5.5° vs 103.2° ± 14.8°, P < 0.05), as did those with operative deliveries compared with those with spontaneous vaginal delivery (97.1° ± 11.5° vs 110.1° ± 14.0°, P < 0.05). The prediction of operative delivery in prolonged second stage of labor by receiver–operating characteristics curve using PAA alone yielded an area under the curve of 0.75. The predicted probability for operative delivery increased as PAA decreased, with an odds ratio of 0.933 for each decrease in angle of 1°.

Conclusion

Our study suggests a correlation between the PAA and mode of delivery in prolonged second stage of labor. This may be used as an adjunctive parameter when considering delivery mode. Copyright © 2012 ISUOG. Published by John Wiley & Sons, Ltd.


INTRODUCTION

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. METHODS
  5. RESULTS
  6. DISCUSSION
  7. REFERENCES

The pubic arch represents the anterior triangle of the pelvic outlet, formed by the interischial tuberosity diameter as the base and the inferior pubic rami as the sides. The angle at the point at which they converge is known as the pubic arch angle (PAA).

In women with a contracted pelvic outlet, the pubic rami converge with a narrow angle, which may cause an obstruction during the second stage of labor. As a consequence, the fetal head is forced posteriorly, towards the posterior triangle, preventing extension of the head and spontaneous completion of vaginal delivery (SVD). Cephalopelvic disproportion occurs when the pelvis does not accommodate for the passage of the fetal head, increasing the incidence of interventional delivery and the risk to both mother and infant[1, 2]; it is responsible for 8% of maternal deaths worldwide[3]. Floberg et al.[4] reported a 0.9% incidence of contracted outlet among 1429 unselected term nulliparae, of whom 85% were delivered by Cesarean section (CS). The incidence of CS increased in proportion to decreasing pelvic outlet capacity. As early as 1938, Caldwell and Moloy[5] noted this relationship between a narrow outlet and instrumental or operative delivery.

Prolonged second stage is a distress point during labor at which it must be decided which procedure would optimize fetal outcome. The use of ultrasound as an auxiliary tool in the management of labor is playing an ever-increasing role in the delivery room[6, 7]. The PAA is a technically simple sonographic parameter that reflects the pelvic outlet form. The main objective of our study was to assess its role in the prediction of delivery mode and its potential contribution to avoiding unnecessary interventional delivery.

METHODS

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. METHODS
  5. RESULTS
  6. DISCUSSION
  7. REFERENCES

During a 12-month period (January 2010–December 2010) we evaluated prospectively women at term (≥ 37 weeks) with a singleton pregnancy and failure to progress in the second stage of labor, as reported in a sister paper regarding head progression distance and mode of delivery, in this issue of the Journal[8]. Prolonged second stage was defined as > 3 h with regional anesthesia or 2 h without regional anesthesia in nulliparous women and > 2 h or 1 h, respectively, in parous women. Informed consent was obtained from all patients, after approval by our hospital's ethics committee.

Ultrasound examinations for the sonographic assessment of PAA were performed by a single operator (Y.G.) using a Voluson I portable ultrasound system (GE Medical Systems, Zipf, Austria) equipped with a 3–5-MHz convex transabdominal transducer. The transducer, covered with a glove and ultrasound gel, was placed transversely on the patient's perineum. Measurements were made between periods of uterine contraction and maternal pushing. The transducer was tilted at 45° (Figure 1), until an image showing the pubic symphysis and the two symmetrical inferior pubic rami was obtained. The angle was measured between the inferior borders of the pubic rami that converge at the middle of the pubic symphysis (Figure 2). The fetal head position was defined by visualization of the orbits anteriorly in occipitoposterior (OP) position, of midline cerebral anatomical landmarks (falx or thalamus) in occipitotransverse (OT) position and of cerebellum or cervical spine in occipitoanterior (OA) position. The data observed, except head position, were not revealed to the staff and did not influence the clinical management. The ultrasound images were stored in the system's hard drive and processed offline. We performed a single measurement of the PAA if an optimal image had been obtained; otherwise, we calculated the average of two to three measurements. Clinical data were collected from each patient's chart.

image

Figure 1. Transperineal ultrasound during labor: transducer position on perineum is transverse, with 45° inclination relative to the long axis of the pubic symphysis (highlighted in yellow).

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image

Figure 2. Transperineal ultrasound images during second stage of labor. (a) Pubic arch angle (PAA), located where pubic rami converge into pubic symphysis. (b) Measurement of PAA is between lines (dotted) running along symmetrical pubic rami which meet at center of pubic symphysis, forming the anterior triangle. (c) Relatively narrow PAA of 96.5° (Caliper 1) in patient who went on to deliver by Cesarean section.

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Statistical analysis was performed using the SPSS 17 statistical package (SPSS, Inc., Chicago, IL, USA) and Microsoft Excel 2007 software (Microsoft Corp, Richmond, CA, USA). We used the ANOVA test (or Kruskal–Wallis test for non-normally distributed variables) to evaluate possible significant differences between clinical fetomaternal characteristics and PAA for the different modes of delivery (SVD, vacuum extraction, forceps extraction and CS). In addition, the correlation between PAA and fetomaternal clinical characteristics and neonatal clinical outcome (Apgar score at 1 and 5 min, umbilical cord arterial pH and number of postnatal hospitalization days) was assessed. A binary logistic regression analysis was performed to evaluate the predictive value of PAA regarding mode of delivery, as a dichotomous dependent variable (SVD vs operative delivery, which included vacuum extraction, forceps extraction and CS deliveries). The results were considered statistically significant if P < 0.05.

RESULTS

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. METHODS
  5. RESULTS
  6. DISCUSSION
  7. REFERENCES

Sixty-two patients in prolonged second stage of labor participated in the study. The mean maternal age was 30.7 (± 4.6) years. The mean gestational age was 39.5 (± 1.4) weeks. Thirty-eight (61.3%) women were nulliparous, 11 (17.7%) had one previous delivery and 13 (21.0%) had two previous deliveries (range of parity, 0–2; median, 0). Four (6.5%) women had undergone a previous CS. The mean body mass index (BMI) was 27.1 (± 4.1) kg/m[2]. Thirty-seven (59.6%) fetuses were in an OA position, 12 (19.3%) were OT and 13 (20.9%) were OP. Seventeen (27.4%) patients underwent SVD, 34 (54.8%) were delivered by instrumental vaginal delivery (30 (48.3%) by vacuum extraction and four (6.5%) by forceps extraction) and 11 (17.7%) were delivered by CS, including one who had failed forceps extraction and three who had failed vacuum extraction. Table 1 summarizes the clinical fetomaternal characteristics according to mode of delivery.

Table 1. Comparison of fetomaternal clinical characteristics according to mode of delivery in 62 patients with prolonged second stage of labor
CharacteristicMode of delivery 
Spontaneous vaginal delivery (n= 17)Vacuum extraction (n= 30)Forceps extraction (n= 4)Cesarean section (n= 11)P
  1. Data are given as mean ± SD or median (range). ANOVA post-hoc multiple comparison tests or Kruskal–Wallis test. *Significant (P < 0.05). BMI, body mass index; GA, gestational age; HC, head circumference.

Maternal age (years)29.4 ± 4.931.4 ± 4.629 ± 1.630.8 ± 4.60.474
Maternal BMI before delivery (kg/m2)27.3 ± 5.326.6 ± 3.527.4 ± 5.128.9 ± 3.90.724
GA at delivery (weeks)39.1 ± 1.239.7 ± 1.338.9 ± 2.339.7 ± 1.60.524
Parity0 (0–2)0 (0–2)0 (0–0)0 (0–2)0.520
Birth weight (g)3203.6 ± 440.83337.1 ± 303.93083.5 ± 627.93278.1 ± 399.10.483
Neonatal HC (mm)347.5 ± 15.2348.64 ± 11.4342.5 ± 14.4302.2 ± 17.90.062
Ischial spine station (0 to 5+)2 (0–3)2 (0–3)3 (2–3)0 (0–2)*< 0.05

The mean PAA was 101.1° (± 13.1°; range, 80°–135°). PAA correlated significantly with mode of delivery, fetal head position and maternal age, while mode of delivery (as a dichotomous variable, i.e. SVD vs operative delivery) correlated significantly only with PAA (Table 2). Comparison of PAA according to fetal head position revealed that the OT group had a statistically significantly smaller PAA compared with the OA group: 94.3° ± 5.5° and 103.2° ± 14.8°, respectively (P = 0.044) (Figure 3a). Further comparison of PAA according to delivery mode found that the SVD group presented a significantly wider mean PAA compared with the other groups (P = 0.023) (Figure 3b). Comparison of SVD with operative deliveries as one group indicated a significant difference in PAA (110.1° ± 14.0° and 97.1° ± 11.5°, respectively, P = 0.001) (Figure 3c). Comparison of the mean PAA of all the vaginal deliveries as one group (102.2° ± 14.2°) with the CS group (96.5° ± 7.1°) revealed a difference of borderline significance (P = 0.057). This statistical elaboration was under-powered (27.4%), due mainly to the low number of cases in the CS group (55 cases was the minimum number required for each group for a power of 80% at a significance level of 5%).

Table 2. Correlation of pubic arch angle and of mode of delivery with fetomaternal clinical characteristics in 62 patients with prolonged second stage of labor
 Pubic arch angleMode of delivery
CharacteristicrPrP
  1. Correlation was assessed as ordinal variables, excluding CS (yes/no), gender (male/female) and mode of delivery (spontaneous vaginal delivery/operative delivery). *Significant correlation at 0.05 level (two-tailed). BMI, body mass index; CS, Cesarean section; HC, head circumference.

Spearman's correlation    
  Previous CS−0.0110.9320.0050.97
  Fetal gender−0.0470.714−0.0170.897
  Fetal head position−0.270.034*0.1260.333
  Mode of delivery0.3890.002*
Pearson's correlation    
  Pubic arch angle−0.2670.036*
  Maternal parity−0.10.438−0.1710.184
  Maternal age−0.2800.027*0.0230.86
  Maternal BMI0.2430.10.0140.916
  Gestational age−0.030.82−0.0690.592
  Birth weight0.1260.3320.120.352
  Neonatal HC−0.1160.3810.0560.663
image

Figure 3. Pubic arch angle (mean ± SD) according to: (a) fetal head position (occipitoanterior (OA), occipitoposterior (OP) or occipitotransverse (OT)); (b) each mode of delivery (spontaneous vaginal (SVD), vacuum extraction (VE), forceps extraction (FE), Cesarean section (CS)); and (c) mode of delivery, grouped as SVD vs all three operative delivery types (OD). Level of significance is indicated for post-hoc multiple comparison tests (ANOVA).

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Binary logistic regression analysis was used to examine the relationships between the different variables (maternal age, gestational age, maternal BMI, parity, previous CS, fetal gender, birth weight, neonatal head circumference, fetal head position and PAA) and the dichotomous outcome of SVD or operative delivery. PAA was the only variable with a significant relation to mode of delivery (P = 0.022); for every unit increase in PAA, there was a weighted odds ratio of 0.933 (Figure 4). The predicted probability for operative delivery by receiver–operating characteristics curve yielded an area under the curve of 0.75 (95% CI, 0.61–0.90) (Figure 5). No spontaneous deliveries were observed among cases with PAA < 90°, irrespective of fetal head position. There was no significant correlation between PAA and neonatal outcome (Table 3).

image

Figure 4. Logistic regression curve showing predicted probability for operative delivery based on pubic arch angle. Odds ratio of operative delivery decreased as pubic arch angle increased and vice versa.

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image

Figure 5. Receiver–operating characteristics curve for pubic arch angle in predicting operative delivery. Area under the curve = 0.75 (P = 0.03).

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Table 3. Pubic arch angle and perinatal outcome: Spearman's correlation coefficient
Perinatal outcomeMean ± SD or median (range)rP
Apgar score: 1 min9 (3–9)−0.1300.319
Apgar score: 5 min10 (9–10)−0.0040.978
Umbilical cord arterial pH7.2 ± 0.4−0.030.727
Duration of hospitalization (days)2.5 ± 0.9−0.1370.296

DISCUSSION

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. METHODS
  5. RESULTS
  6. DISCUSSION
  7. REFERENCES

A contracted pelvic outlet is often associated with midpelvic contraction, due to their anatomical continuity. The narrower the pubic arch, the more the engaged fetal head is pressed against the posterior triangle of the pelvic outlet. This results in an increased likelihood of OP position[9], longer second-stage duration[10] and higher incidence of instrumental delivery and, consequently, postpartum anal sphincter trauma and incontinence[11, 12]. The accuracy of clinical evaluation to detect pelvic outlet contraction is limited; according to Floberg et al., it is around 50%[13]. The use of X-ray pelvimetry has been found to double the rate of CS, without significant impact on perinatal outcome[14]; it has now been abandoned almost completely in the routine management of labor.

In this preliminary study, we used a simple sonographic method during prolonged second stage of labor as an indicator for successful SVD. The predicted probability for operative delivery increased from 0.24 for a PAA of 135° to 0.93 for a PAA of 80°. There was no significant influence of the PAA on the immediate postnatal outcome.

The results of previous research indicate that older women are more likely to have a sphincter tear[15]. The inverse relationship between maternal age and PAA found in our study may indicate that this factor contributes to traumatic pelvic injury, which has been found to be more likely in older laboring patients. It is possible that the narrow anterior triangle would increase the pressure at the posterior triangle, leading to anal sphincter injury.

Our present research is the first to demonstrate a correlation between PAA and OT. This is not surprising, as a narrow angle obstructs the head from successful internal rotation. In the OT group, spontaneous delivery did not occur, with all cases requiring intervention. This observation is especially important when considering whether to allow extension of prolonged second stage in order to enable an opportunity for spontaneous delivery.

The main limitations of the study were: 1) the relatively low number of cases, especially in the CS subgroup, which caused the study comparing SVD and CS groups to be underpowered; 2) the lack of information regarding intra- or interobserver variability of this new biometric parameter.

The search for objective parameters to assist in clinical decision-making in the delivery ward will continue. It is premature to rely solely on PAA or other sonographic parameters, as the labor mechanism involves various confounding factors in addition to the relationship between the bony pelvis and the fetal head. Moreover, our findings may not be applicable in other units with different management of labor and CS rates. Although our study suggests that PAA may be a predictive parameter that could improve decision-making regarding operative delivery in cases of obstructed labor, we feel that a larger prospective study, including evaluation of reliability of its measurement, in the first stage of labor as well as the second, is required to confirm the potential role of PAA in the clinical management of labor.

REFERENCES

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. METHODS
  5. RESULTS
  6. DISCUSSION
  7. REFERENCES
  • 1
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    World Health Organization. The World Health Report 2005: making every mother and child count. WHO, Geneva, Switzerland, 2005. http: //www.who.int/whr/2005/en/index.html [Accessed 5 April 5 2010].
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    Floberg J, Belfrage P, Ohlsen H. Influence of pelvic outlet capacity on labour. A prospective pelvimetry study of 1429 unselected primiparas. Acta Obstet Gynecol Scand 1987; 66: 121126.
  • 5
    Caldwell WE, Moloy HC. Anatomical variations in the female pelvis: their classification and obstetrical significance. Proc R Soc Med 1938; 32: 130.
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    Molina FS, Nicolaides KH. Ultrasound in labor and delivery. Fetal Diagn Ther 2010; 27: 6167.
  • 7
    Kalache KD, Dückelmann AM, Michaelis SA, Lange J, Cichon G, Dudenhausen JW. Transperineal ultrasound imaging in prolonged second stage of labor with occipitoanterior presenting fetuses: how well does the ‘angle of progression’ predict the mode of delivery? Ultrasound Obstet Gynecol 2009; 33: 326330.
  • 8
    Gilboa Y, Kivilevitch Z, Spira M, Kedem A, Katorza E, Moran O, Achiron R. Head progression distance in prolonged second stage of labor: relationship with mode of delivery and fetal head station. Ultrasound Obstet Gynecol 2013; 41: 436441.
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    Floberg J, Belfrage P, Ohlsén H. Influence of pelvic outlet capacity on labor: a prospective pelvimetry study of 1429 unselected primiparas. Acta Obstet Gynecol Scand 1987; 66: 127130.
  • 10
    Frudinger A, Halligan S, Spencer JA, Bartram CI, Kamm MA, Winter R. Influence of the subpubic arch angle on anal sphincter trauma and anal incontinence following childbirth. BJOG 2002; 109: 12071212.
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    Donnelly V, Fynes M, Campbell D, Johnson H, O'Connell PR, O'Herlihy C. Obstetric events leading to anal sphincter damage. Obstet Gynecol 1998; 92: 955961.
  • 12
    Dudding TC, Vaizey CJ, Kamm MA. Obstetric anal sphincter injury: incidence, risk factors, and management. Ann Surg 2008; 247: 224237.
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    Floberg J, Belfrage P, Carlsson M, Ohlsen H. The pelvic outlet. A comparison between clinical evaluation and radiologic pelvimetry. Acta Obstet Gynecol Scand 1986; 65: 321326.
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    Pattinson RC. Pelvimetry for fetal cephalic presentations at term. Cochrane Database Syst Rev 2000; (2): CD000161.
  • 15
    Fitzgerald MP, Weber AM, Howden N, Cundiff GW, Brown MB. Risk factors for anal sphincter tear during vaginal delivery. Obstet Gynecol 2007; 109: 2934.