Cephalopelvic disproportion (CPD) in labour occurs when there is a mismatch between the size of the fetus and the dimensions of the maternal pelvis. The factors which mainly influence the outcome of the delivery can be summarised as the three “Ps” of the labour: passageway, passenger, and power of the uterus [
The purpose of this observational cohort study was to evaluate whether pelvic measurements, especially pelvic outlet, displayed any association with operative vaginal deliveries and the duration of the second stage of the delivery.
This retrospective study was approved by the Ethical Committee of North-Carelian Central Hospital. It investigated Caucasian women, that had been examined by X-ray or MRI pelvimetry during 2000–2008 in North-Carelian Central Hospital. The patients were sent to the hospital antenatal unit from their general health care. Eligibility criteria included that pelvimetric and fetal measurements had been recorded. In the operative delivery group, the criteria were as follows. There were no signs of fetal distress in cardiotocography, inertia was not diagnosed, and there was no malpresentation. Originally, 915 women were screened for possible inclusion, but 429 women were excluded because of breech presentation. A total of 486 patients with the fetus in the cephalic presentation were screened in the study, but those 234 women that went through elective or acute Cesarean section were excluded from the analysis. The clinical indication for pelvimetry was breech presentation, or if the fetus was in cephalic presentation, the indication was suspected cephalopelvic disproportion in clinical examination. The findings that referred to CPD in clinical examination were clinically small pelvis, unengaged presentation, or suspected macrosomia. Pelvimetric measurements were found in all patients, as required by the inclusion criteria. There were 252 participants with fetal cephalic presentation delivered vaginally, of whom 184 women delivered spontaneously and 68 women went through operative vaginal delivery with vacuum extraction. Of this latter group of women, in 26 patients, the vacuum extraction was undertaken primarily because of fetal distress and inertia, and these patients were excluded from the final analysis, leaving 42 women in the operative vaginal delivery group. Thus, the total number of participants evaluated in the final stage of this study was 226.
The obstetric and radiologic data were collected from patients’ medical records by the author (UK) and transferred into a commercially available worksheet (Excel, Microsoft 2003, Ireland). The patients were numbered for identification in the order of their pelvimetric examination date. The following pelvimetric parameters were recorded: in the pelvic inlet, anteroposterior (conjugata vera) and transverse diameters and in outlet, interspinous diameter and sagittal diameter from the surface of the pubic symphysis to the surface of the sacrum measured at the spinous level. Pelvic inlet and outlet circumferences were calculated from the pelvic anteroposterior and transverse diameters using the formula (
In this evaluation of the diagnostic accuracy of the pelvimetry in vaginal deliveries, the mode of the vaginal delivery was chosen to represent the reference standard. The target condition was spontaneous vaginal delivery. Patients were divided into subgroups according to the size of the fetus and also by the parity to evaluate the variability reflecting differences in patient groups.
For statistical analysis, we used SPSS 17.0 (SPSS Inc., 2009, Chicago, IL, USA). Chi-square test was used to investigate the statistical significances. Receiver operating characteristic (ROC) [
Figure
Flow chart of the patients and the reasons why there had been a consultation about the mode of delivery.
Demographic data of the patients (
Spontaneous vaginal delivery
Operative vaginal delivery
Patients
184
42
Age ± SD (min.–max.) years
28.3 ± 4.7 (19–40)
26.8 ± 4.7 (18–37)
0.75
Parity, nulliparous/multiparous
74/110
33/9
<0.01
Weight ± (min.–max.) kg
68.1 ± 15.5 (43–150)
67.6 ± 16.1 (46–103)
0.87
Height ± (min.–max.) cm
164 ± 5.6 (148–177)
163 ± 5.5 (154–176)
0.92
Body mass index ± (min.–max.)
25.4 ± 5.3 (17–52)
25.4 ± 5.5 (18–39)
0.98
Infant weight ± (min.–max.) g
3750 ± 530 (2210–5120)
3760 ± 380 (2915–4680)
0.80
Labour induction
136 (74%)
30 (71%)
0.78
Labour augmentation
141 (77%)
40 (96%)
0.02
Patients were further subdivided into two subgroups according to the infant’s weight and the mode of delivery. The maternal pelvic inlet and outlet sizes and duration of the first and second stages of the delivery by the mode of delivery in infant weight subgroups are shown in Table
Descriptive data of the patients subdivided according to the route of delivery and infant weight.
Route of delivery | Inlet mm mean (SD) | Outlet mm mean (SD) | First stage of the delivery, minutes mean (SD) | Second stage of the delivery, minutes mean (SD) | Infant weight g mean (SD) | Apgar 1-minute mean (SD) |
---|---|---|---|---|---|---|
All ( |
401 (22) | 361 (20) | 519 (284) | 50 (38) | 3750 (509) | 8.6 (1.1) |
Spontaneous vaginal delivery ( |
402 (22) | 362 (20) | 483 (282) | 40 (31) | 3750 (534) | 8.6 (0.95) |
Operative vaginal delivery |
399 (21) | 357 (18) | 615 (280) | 94 (31) | 3760 (378) | 8.2 (1.4) |
|
0.23 | 0.84 | 0.66 | <0.01 | 0.80 | 0.06 |
| ||||||
Infant weight < 3700 g ( |
392 (19) | 351 (17) | 521 (303) | 49 (33) | 3300 (291) | 8.5 (1.0) |
Spontaneous vaginal delivery ( |
391.5 (18) | 352 (17) | 515 (309) | 40 (29) | 3270 (302) | 8.7 (0.7) |
Operative vaginal delivery ( |
393 (19) | 347 (15) | 543 (282) | 85 (27) | 3430 (193) | 7.8 (1.8) |
|
0.23 | 0.93 | 0.56 | 0.01 | 0.61 | <0.01 |
| ||||||
Infant weight ≥ 3700 g ( |
409 (20) | 370 (18) | 501 (267) | 51 (41) | 4120 (318) | 8.6 (1.1) |
Spontaneous vaginal delivery ( |
410 (20) | 370 (18) | 457.1 (256) | 40 (34) | 4130 (327) | 8.6 (1.1) |
Operative vaginal delivery ( |
402 (22) | 365.5 (16) | 675 (270) | 102 (32) | 4030 (259) | 8.6 (0.9) |
|
0.37 | 0.31 | 0.65 | <0.01 | 0.78 | 0.82 |
SD: standard deviation.
The receiver operating characteristic curve analysis for pelvic inlet and outlet as a diagnostic test for the mode of vaginal delivery is shown in Figures
ROC curves of the maternal pelvic inlet and outlet and the mode of the vaginal delivery. Spontaneous vaginal delivery was chosen as the reference result. (a) ROC curve for maternal inlet. The area under curve is 0.566 with the
The main finding of this study was that the maternal bony pelvic dimensions displayed virtually no correlation to the need for operative vaginal deliveries. The indications for intervention in vaginal deliveries were chosen on clinical grounds as evidenced by the fact that there was an association between the duration of the second stage of the delivery and the size of the pelvic outlet. If the delivery had reached the second stage, it was probable that the uterine “power” played a more significant role in the overall outcome than either the “passageway” or the “passenger” [
The pelvimetry was performed in most of the patients because of suspected disproportion, or an intervention had been required in a previous labour. Of those patients that had previous CS and were now exposed to the trial of labour, over 80% delivered spontaneously, and less than 20% required an operative vaginal delivery. This is in agreement with previous studies [
There were no statistically significant differences between the size of the maternal inlet or outlet in the spontaneous and the operative vaginal delivery groups. When patients were divided into subgroups according to the infant weight, the maternal inlet was 4.7% and the outlet was 5.1% larger in the infant weight ≥3700 g subgroup among those who delivered spontaneously compared to those vaginally assisted. The duration of the first stage of the delivery was longer in the smaller infant group, whereas the second stage was shorter than in the larger infant group. In the two delivery subgroups, the duration of the second stage of the delivery was significantly longer in operative vaginal delivery group than in spontaneous vaginal delivery group. The Apgar scores were acceptable in all delivery groups referring to the fact that both spontaneous and operative vaginal deliveries were uncomplicated and severe shoulder dystocia was not present. However, the Apgar 1-minute scores were lower in the operative vaginal delivery group than in spontaneous vaginal delivery group when infant weight was <3700 g. These results refer to the fact that operative vaginal delivery increases the time of the second stage of the delivery and decreases the Apgar 1-minute scores.
The ROC curve analysis for maternal pelvic inlet and outlet revealed that both inlet and outlet had only a fair prognostic value in predicting the mode of the vaginal delivery. The poor predictive value of pelvimetry to predict protracted labor is a well-known fact from previous studies, whereas the evidence on the need of vaginal operative deliveries is less extensively evaluated [
The study had some limitations. The data did not reveal in detail the fluency of the operative deliveries. As mentioned earlier, no severe dystocia was present as reflected in the one-minute Apgar scores. Therefore, it was not possible to evaluate the influence of the pelvic dimensions on the severe dystocia. For that kind of study, the cohort examined here study is too small due to the rare incidence of severe dystocia [
In conclusion, our study revealed that maternal bony pelvic dimensions, either pelvic inlet or outlet, were not associated with the need for operative vaginal deliveries. It was more likely that other factors related to the maternal perineal soft tissue, maternal resources, and the passenger were the reasons leading to operative vaginal deliveries. Subsequently, we cannot recommend that caregivers use pelvimetric measurements to predict the outcome of the second stage of the labour. Observational studies with larger cohorts would be needed, if one wished to investigate whether the maternal bony pelvic size has any effect on severe dystocia. In addition, the three-dimensional shape of the bony pelvis and the soft tissues are worth considering in future studies.