Pelvic inlet area is associated with birth mode

Abstract

Introduction: To determine whether a pelvis is wide enough for spontaneous delivery has long been the subject of obstetric research. A number of variables have been proposed as predictors, all with limited accuracy. In this study, we use a novel three-dimensional (3D) method to measure the female pelvis and assess which pelvic features influence birth mode. We compare the 3D pelvic morphology of women who delivered vaginally, women who had cesarean sections, and nulliparous women. The aim of this study is to identify differences in pelvic morphology between these groups.

Material and methods: This observational study included women aged 50 years and older who underwent a CT scan of the pelvis for any medical indication. We recorded biometric data including height, weight, and age, and obtained the obstetric history. The bony pelvis was extracted from the CT scans and reconstructed in three dimensions. By placing 274 landmarks on each surface model, the pelvises were measured in detail. The pelvic inlet was measured using 32 landmarks. The trial was registered at the German Clinical Trials Register DRKS (DRKS00017690).

Results: For this study, 206 women were screened. Exclusion criteria were foreign material in the bony pelvis, unknown birth mode, and exclusively preterm births. Women who had both a vaginal birth and a cesarean section were excluded from the group comparison. We compared the pelvises of 177 women between three groups divided by obstetric history: vaginal births only (n = 118), cesarean sections only (n = 21), and nulliparous women (n = 38). The inlet area was significantly smaller in the cesarean section group (mean = 126.3 cm² ) compared with the vaginal birth group (mean = 134.9 cm² , p = 0.002). The nulliparous women were used as a control group: there was no statistically significant difference in pelvic inlet area between the nulliparous and vaginal birth groups.

Conclusions: By placing 274 landmarks on a pelvis reconstructed in 3D, a very precise measurement of the morphology of the pelvis is possible. We identified a significant difference in pelvic inlet area between women with vaginal delivery and those with cesarean section. A unique feature of this study is the method of measurement of the bony pelvis that goes beyond linear distance measurements as used in previous pelvimetric studies.

Keywords: birth mode; female pelvic morphology; fetopelvic disproportion; geometric morphometrics; pelvic inlet area; pelvimetry.

FIGURE 1

(A) Three‐dimensional landmarks and semi‐landmarks. The 274 landmarks (83 anatomical landmarks in red, 191 semi‐landmarks in magenta) are shown on the mean female pelvis in our sample in anterior, superior, and lateral views. (B) Inlet landmarks. The 32 inlet landmarks shown on the mean female pelvis.

FIGURE 2

Flowchart illustrating case selection for this study; indicated are case numbers (n) and, for the final group comparison, the percentages (%) for each group. In all, 177 women were included in the final group comparison between cesarean delivery (n = 21), vaginal delivery (n = 118), and nulliparous women (n = 38).

FIGURE 3

Distributions of inlet area. Left panel: Vaginal birth group (orange) vs cesarean section group (blue); Right panel: Vaginal birth group (orange) vs nulliparous group (magenta). Mean inlet area was 7% smaller in the cesarean section group compared with the vaginal birth group.

FIGURE 4

Pelvic inlet area plotted against body height for all three groups (177 women), indicating that tall women had a more spacious pelvic inlet, on average. Colors indicate birth mode groups: vaginal birth group (orange), cesarean section group (blue), nulliparous group (magenta).

FIGURE 5

Maternal height distributions for the vaginal birth group (orange) and the cesarean section group (blue).