Objective
The purpose of this study was to determine whether x-ray measures of the mid pelvis can be used to predict cesarean delivery.
Study Design
Women were enrolled prospectively; x-ray pelvimetry was performed after delivery; the readers were blinded to the outcome. Groups were determined by mid pelvis measures (transverse diameter, anteroposterior diameter, and circumference ≤10th percentile. The primary outcome was cesarean delivery. Univariable, stratified, and multivariable analyses were performed to estimate the effect of mid pelvis measures on cesarean delivery. Receiver operator characteristics curves were created to estimate the predictive value of mid pelvis measures of cesarean delivery.
Results
Four hundred twenty-six women were included. Subjects with anteroposterior diameter or circumference ≤10th percentile were at greater risk of cesarean delivery (risk ratio for anteroposterior diameter, 4.8; 95% confidence interval, 3.9–5.8; risk ratio for circumference ≤10th percentile, 3.8; 95% confidence interval, 3.1–4.8). Transverse diameter ≤10th percentile was not associated with an increased risk of cesarean delivery. The area under the receiver operator characteristics curves for anteroposterior diameter, circumference ≤10th percentile, and transverse diameter were 0.88, 0.85, and 0.69, respectively ( P < .01).
Conclusion
Simple radiographic measures of the mid pelvis on x-ray can provide a useful adjunct to clinical information in the determination of who should attempt a vaginal delivery.
Cephalopelvic disproportion and arrest of dilation are cited as the indication in almost 50% of cesarean deliveries. Radiographic pelvimetry to predict cephalopelvic disproportion has fallen out of favor because of its poor predictive value and the perception that it provided little information beyond the clinical examination. Additionally, randomized controlled trials demonstrated that women who underwent radiographic pelvimetry had an increased risk of cesarean delivery. Given these findings, radiographic pelvimetry is no longer applied in clinical settings. Instead, women are generally allowed a trial of labor to test whether or not their pelvis is “adequate.”
However, the studies of radiographic pelvimetry had a limited number of patients, were subject to bias because of lack of blinding, and used arbitrary cutoff point of “adequate” vs “contracted” pelvis. Some “critical” limits identified range from 10–11.5 cm in the anteroposterior diameter (APD), and some studies used the sum of the measurements. Furthermore, although the best test of the adequacy of a pelvis is a trial of labor, women who have had a previous cesarean delivery and no previous vaginal delivery may have already demonstrated that their pelvis was not adequate. Because most morbidity from a trial of labor after cesarean delivery (TOLAC) is due to a failed TOLAC, it would be ideal to identify women who will have a failed TOLAC before their attempt.
Therefore, we sought to determine the utility of radiographic measures of the mid pelvis to predict cesarean delivery.
Materials and Methods
This is a secondary analysis of a multicenter prospective cohort study that was designed to evaluate the use of the fetal pelvic index for the prediction of cesarean delivery. Institutional review board approval was obtained from all sites. Subjects were recruited before labor at >36 weeks’ gestation at 2 sites in the University of Pennsylvania health system: Pennsylvania Hospital and the Hospital of the University of Pennsylvania. Women were eligible if they had a viable singleton intrauterine pregnancy at >36 weeks’ gestation and vertex presentation and planned to attempt vaginal delivery. Women were excluded if they had a multiple gestation; breech presentation; planned cesarean delivery; previous low vertical, classic, or unknown cesarean scar, or other maternal contraindications to vaginal delivery.
X-ray pelvimetry was obtained after delivery with the Colcher-Sussman technique. Pelvimetry was performed after delivery to avoid exposure of the fetus to ionizing radiation for research purposes; these measurements are not impacted significantly by the antepartum vs postpartum state. With this method, a lateral view of the pelvis was obtained with the patient on her side with knees and thighs semiflexed; an anteroposterior view of the pelvis was obtained with the patient flat on her back with her knees and thighs semiflexed. In the lateral view, the APD of the mid pelvis was measured from S3 to the pubic symphysis. In the anteroposterior view, the transverse diameter (TD) of the mid pelvis is measured at the level of the ischial spines. The mean circumference of the mid pelvis was calculated from the APD and TD measurements with the following formula: (APD + TD) × π/2. Each measure was determined by 2 independent radiologists who were blinded to mode of delivery, and the average measurement was used. In cases in which the measurements differed by >5%, the radiologists met and discussed the case and agreed on the measurement to be used.
The exposure group was defined by APD, TD, and mean circumference <10th percentile. The 10th percentile was chosen a priori as a statistical-based, but clinically arbitrary, cutoff point. The primary outcome was cesarean delivery in labor. All cesarean deliveries were considered as cephalopelvic disproportion is a difficult and frequently subjective diagnosis. A planned secondary analysis was performed that defined the exposure as mid pelvis measures <5th percentile. Finally, an analysis was performed to determine whether mid pelvis measures of >90th percentile could predict successful vaginal delivery.
Demographics and historic factors for the 2 study groups were compared in a univariable analysis with the use of unpaired t tests for continuous variables and χ 2 or Fisher exact tests for categoric variables. Univariable analyses were used to estimate the effect of mid pelvis measures <10th percentile on the risk of cesarean delivery. Stratified analyses were used to identify potentially confounding variables in the outcome-exposure association. Multivariable logistic regression models were then developed to better estimate the independent effect of mid pelvis measures that were <10th percentile on the risk of cesarean delivery. Potential confounders were added to the model based on biologic plausibility and statistical significance in the unadjusted analysis. Receiver operating characteristic curves were created to determine the predictive value of radiographic measures of the mid pelvis for cesarean delivery.
Results
Of 652 women in the cohort, 426 women met the inclusion criteria (3 were excluded for gestational age at delivery less than 36 weeks; 1 was excluded for multifetal gestation; 6 were excluded for breech presentation; 46 were excluded for elective cesarean delivery, and 170 were lost to follow up because they did not deliver at a participating institution). The 5th, 10th, and 90th percentiles for the APD, TD, and mean circumference <10th percentile are shown in Table 1 . Table 2 shows baseline demographic data for subjects with a mid pelvis mean circumference > or < the 10th percentile; the 2 groups are similar with respect to age, gravidity, previous vaginal delivery, previous cesarean delivery, birthweight, and labor type. Subjects with a mid pelvis mean circumference <10th percentile were more likely to be black or Asian.
| Percentile | Anteroposterior diameter, cm | Transverse diameter, cm | Circumference, cm |
|---|---|---|---|
| 5th | 9.0 | 8 | 29.0 |
| 10th | 9.5 | 9 | 29.8 |
| Median | 11.5 | 10 | 34.1 |
| 90th | 12.5 | 12 | 37.3 |
| Variable | Mid pelvis circumference | P value | |
|---|---|---|---|
| ≤10th percentile (n = 47) | >10th percentile (n = 379) | ||
| Maternal age, y a | 23.2 ± 5.2 | 23.9 ± 6.1 | .22 |
| Gravidity, n a | 1.6 ± 0.7 | 1.6 ± 0.9 | .43 |
| Race, n (%) | < .01 | ||
| White | 3 (6.4) | 107 (28.2) | |
| Black | 39 (83.0) | 219 (57.8) | |
| Hispanic | 0 | 18 (4.8) | |
| Asian | 3 (6.4) | 12 (3.2) | |
| Labor type, n (%) | .16 | ||
| Spontaneous | 7 (15.9) | 110 (29.3) | |
| Induced | 12 (27.3) | 79 (21.1) | |
| Augmented | 25 (56.8) | 186 (49.6) | |
| Previous cesarean delivery | 11 (23.4) | 49 (12.9%) | .08 |
| Previous vaginal delivery | 0 | 4 (1.1) | .45 |
| Birthweight, g b | 3445 (3172-3681) | 3354 (3090-3650) | .19 |
A TD of the mid pelvis <10th percentile was not associated with an increased risk of cesarean delivery (positive predictive value, 45.5%; adjusted odds ratio, 2.0; 95% confidence interval [CI], 1.0–3.8; Table 3 ). A TD of <5th percentile was also not associated with increased risk of cesarean delivery (positive predictive value, 54.2%; adjusted odds ratio, 2.0; 95% CI, 0.7–5.5). However, an APD of <10th percentile was associated strongly with cesarean delivery; 98% of women with an APD of <9.5 cm required cesarean delivery (positive predictive value, 98%; adjusted odds ratio, 112.7; 95% CI, 14.9–854.1). Of the 28 subjects with an APD <5th percentile, all were delivered by cesarean (positive predictive value, 100%). The sensitivity of an APD of <10th percentile to detect subjects who require cesarean delivery was low at 39%; however, the specificity of the test was 99.7%.
| Exposure group | Cesarean delivery, n (%) a | Risk ratio (95% CI) | Adjusted odds ratio (95% CI) b | P value |
|---|---|---|---|---|
| Transverse diameter | ||||
| <10th (≤9.0 cm; n = 77) | 35 (45.5) | 1.7 (1.3–2.3) | 2.0 (1.0–3.8) | .04 |
| >10th (>9.0 cm; n = 349) | 92 (26.3) | Reference | Reference | |
| <5th (≤8.0 cm; n = 24) | 13 (54.2) | 1.9 (1.3–2.8) | 2.0 (0.7–5.5) | .19 |
| >5th (>8.0 cm; n = 402) | 114 (28.4) | Reference | Reference | |
| Anteroposterior diameter | ||||
| <10th (≤9.5 cm; n = 51) | 50 (98.0) | 4.8 (3.9–5.8) | 112.7 (14.9–854.1) | < .01 |
| >10th (>9.5 cm; n = 375) | 77 (20.5) | Reference | Reference | |
| <5th (≤9.0 cm; n = 28) | 28 (100) | 4.0 (3.4–4.8) | — c | — |
| >5th (>9.0 cm; n = 398) | 99 (24.9) | Reference | — c | |
| Circumference | ||||
| <10th (≤29.8 cm; n = 47) | 41 (87.2) | 3.8 (3.1–4.8) | 19.3 (6.9–53.6) | < .01 |
| >10th (>29.8 cm; n = 379) | 86 (22.7) | Reference | Reference | |
| <5th (≤29.0 cm; n = 20) | 19 (95.0) | 3.6 (3.0–4.3) | — c | — |
| >5th (>29.0 cm; n = 406) | 108 (26.7) | Reference | — c |
a The percentage of cesarean deliveries represents the positive predictive value (<10th, <5th) or the negative predictive value (>10th, >5th) of the pelvic measurement
b Adjusted for previous cesarean delivery, labor induction
A mid pelvis mean circumference of <10th percentile was also associated with an increased risk of cesarean delivery (positive predictive value, 87.2%; adjusted odds ratio, 19.3; 95% CI, 6.9–53.6) and a specificity of 98%. A mean circumference of <5th percentile had 99.7% specificity and a 95% positive predictive value for cesarean delivery.
Table 4 lists the mode of delivery and indication for cesarean delivery by APD <10th percentile. Only one of the women (2%) with APD <10th percentile delivered vaginally. Vacuum and forceps were not used successfully in women with APD of <10th percentile, as opposed to a 10.5% operative vaginal delivery rate in women with APD >10th percentile. The indication for cesarean delivery did not vary significantly by group; approximately one-half of the women with APD of >10th percentile had an arrest disorder specified as their indication for cesarean delivery.
| Variable | Mid pelvis anteroposterior diameter, n (%) | P value | |
|---|---|---|---|
| ≤10th percentile (n = 51) | >10th percentile (n = 375) | ||
| Mode of delivery | < .01 | ||
| Normal vaginal | 1 (2.0) | 258 (68.8) | |
| Vacuum | 0 | 27 (7.2) | |
| Forceps | 0 | 13 (3.5) | |
| Cesarean | 50 (98.0) | 76 (20.3) | |
| Cesarean delivery indication | .72 | ||
| Arrest disorder | 27 (54.0) | 44 (57.1) | |
| Nonreassuring fetal status | 12 (24.0) | 14 (18.2) | |
| Other | 11 (22.0) | 19 (24.7) | |
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