Dynamic External Pelvimetry Test in Third Trimester Pregnant Women: Shifting Positions Affect Pelvic Biomechanics and Create More Room in Obstetric Diameters

Abstract

Dystocia in labor is still a clinical challenge. The "contracted pelvis" is the absence of pelvic mobility, which leads to fetal-pelvic disproportion, obstructed labor, and operative delivery. Maternal pelvis biomechanics studies by high technological techniques have shown that maternal shifting positions during pregnancy and labor can create more room in the pelvis for safe delivery. The external and internal pelvic diameters are related. The present study aims to evaluate the external obstetric pelvic diameters in shifting positions using a clinical technique suitable for daily practice in every clinical setting: the dynamic external pelvimetry test (DEP test). Seventy pregnant women were recruited, and the obstetric external pelvic diameters were measured, moving the position from kneeling standing to "hands-and-knees" to kneeling squat position. Results showed modification of the pelvic diameters in shifting position: the transverse and longitudinal diameters of Michaelis sacral area, the inter-tuberosities diameter, the bi-trochanters diameter, and the external conjugate widened; the bi-crestal iliac diameter, the bi-spinous iliac diameter, and the base of the Trillat's triangle decreased. The test showed good reproducibility and reliability. Linear correlations were found between diameters and between the range of motion of the diameters. The maternal pelvis is confirmed to modify the diameters changing its tridimensional shape. The pelvic inlet edge's inclination is inferred to be modified, facilitating the fetal descend. The pelvic outlet enlarged the transverse diameter, facilitating birth. The DEP test estimates the pelvic diameters' modification with postural changes, as magnetic resonance (MR) and computational biomechanics studies have demonstrated.

Keywords: biomechanics; childbirth; dystocia; labor; obstetrics; pelvimetry; pelvis; pregnancy.

Conflict of interest statement

Marco Siccardi recognized that he has an intellectual property right for the BMK instrument, CE patented. Co-authors don't have to disclose any patents or copyrights

Copyright © 2021, Siccardi et al.

Figures

Figure 1. The diameters of the Dynamic External Pelvimetry test.

A: the transverse diameter of the sacral area. B: the longitudinal diameter of the sacral area. C: the longitudinal hemi-diameter of the sacral area. D: the base of Trillat's triangle. E: the iliac bi-spinous diameter. F: the iliac bi-crestal diameter. G: the bi-trochanters diameter. H: the ischial inter-tuberosities diameter. I: the external obstetric conjugate.

Figure 2. DEP test: shifting positions procedure.

A, D: p1, straight-leg positions. A: standing kneeling position. D: supine position. B, E: p2, bent-leg positions. B: "all-fours" position. E: bent-leg supine position. C: p3, kneeling squat position.

Figure 3. Graphic representation of the values of the pelvic diameter in shifting positions

Data are shown in millimeters as mean and standard deviation, minimum and maximum. The paired t-test was statistically significant for all the diameters, not in all positions. p1: straight-leg position. p2: bent-leg position. p3: squat position. *p-value

Figure 4. Intra-observer and inter-observer agreement

A, B: Intra-observer Bland-Altman plot showing mean difference ±2 standard deviation (SD) values (v). C, D: inter-observer Brand-Altman plot showing mean difference ±2 SD values (v). ROM: the range of motion.

Figure 5. Pelvic anteroposterior diameters in shifting positions.

A, p1: straight-leg position. B, p3: squat position. Red line: external conjugate measured in straight-leg position. Yellow line: obstetric conjugate measured in the straight-leg position. Grey line: midlet anteroposterior diameter measured in the straight-leg position. Blue line: outlet anteroposterior diameter measured in straight-leg position. Black line: anterior sacral promontory inclination, angle respect the lumbar spine anterior line. Orange line: lumbar spine anterior line. Orange arrows: the longitudinal hemi-diameter of the Michaelis area. Orange angle: the angle between the lumbar spine and the external conjugate, according to Desseauve et al. [6]. Green dots: pelvic movement fulcra (lumbar-sacral posterior zygoapophyseal joint and hip joint). Green curved arrow: the direction of the sacral base, according to Hemmerich et al. [5]. Graphic images of the pelvis sagittal sections show the biodynamics of the anteroposterior diameters' measurements from the present study and literature cited in the text: the external conjugate, the midlet, and outlet anteroposterior diameter increase. The obstetric conjugate decreases from the straight-leg to the squat position. The lumbar-sacral junction flexion increases the longitudinal hemi-diameter of the Michaelis area, and the sacral promontory flattens. The external conjugate becomes close to perpendicular to the lumbar spine anterior line, approaching the theoretical best birth position.

Figure 6. Pelvic diameters and inlet edge modification to the squat positions

A, B: figures show the direction of pelvic diameters' measurement change to the squat position. A: anterior view of the pelvis. B: posterior view of the pelvis. Blue arrows: subpubic arch angle and inter-tuberosities diameter. Dark grey arrows: bi-trochanter diameter. Light blue arrows: iliac bi-crestal diameter. Green arrows: ischial bi-spinous diameter. Yellow arrows: iliac bi-spinous diameter. Dark red arrows: the base of Trillat's triangle. Grey arrows: longitudinal hemi-diameter of Michaelis sacral rhombus. Orange arrows: transverse diameter of Michaelis sacral rhombus. Red dots: the posterior superior iliac spines. Red line: pelvic inlet edge. C, D: hipbones orientation in the frontal plane. C: frontal section of the ischium-iliac bones in starting position. D: frontal section of the ischium-iliac bones in the squat position. Red line: pelvic inlet. Black arrows: arcuate lines' edge inclination. Blue arrows: inter-tuberosities diameter modification direction to the squat position. Light blue arrows: bi-crestal diameter modification direction to the squat position. Arrowed green line: midpelvic transverse measure in starting position. Arrowed blue line: pelvic outlet transverse measure in starting position.