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

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.