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The pelvic inlet, also referred to as the superior pelvic aperture or upper pelvic narrow, is the anatomical limit between the true pelvis inferiorly and the false pelvis superiorly. There are morphologic, genetic, and hormonal differences related to reproduction that differentiate the male and female pelvis. In obstetrics, the pelvic inlet is the entrance to the birth canal. The fetal cephalic extremity must position itself and adapt adequately to compare the smaller diameter with the largest diameter of the space delimited by the anatomical line of the maternal pelvic inlet. The shape of the inlet depends on the general shape of the pelvis, according to the traditional classification of Caldwell and Moloy.[1] The dimensions of its anteroposterior, oblique, and transverse diameters vary according to the morphological type of the pelvis. The proportions of the shape of the internal pelvic spaces correspond to the proportion of the sacral area of Michaelis. Radiological evidence shows that the subject's posture changes the intrapelvic space. The position taken by the subject influences the values of the transversal and anterior-posterior diameters. This evidence is instrumental in facilitating fetal entry into the true pelvis and favoring the dilating phase of labor.[2] Evaluating the diameters of the endopelvic spaces and their adaptability (mobility) would be beneficial for diagnosing the "contracted pelvis" and for avoiding the adverse effects on maternal and newborn health that protracted labor and operative birth can entail.