Summary
Highlights
The video introduces the obstetric pelvis as the secret passage babies use for birth. It is an osseous structure at the base of the spine, a bone girdle between the vertebral column and lower limbs. Obstetricians evaluate its suitability for vaginal delivery, with C-sections as an alternative if concerns arise about its size or shape.
The pelvis is formed by the union of several bones: two iliac bones anteriorly and laterally, and the sacrum and coccyx posteriorly. Each iliac bone results from the fusion of three bones: the ilium, pubis, and ischium. These bones are connected by four articulations, three of which are symphyses: the pubic symphysis anteriorly, the sacroiliac symphyses posteriorly and laterally, and the sacrococcygeal articulation posteriorly and inferiorly.
The obstetric pelvis is compared to a cylinder and described in three main parts: the superior strait (D3 supérieur), the pelvic excavation (D3 moyen), and the inferior strait (D3 inférieur).
The superior strait is the widest of the three spaces, located in the anterior and superior part of the pelvis. It provides ample space for the fetal head's passage during delivery. Its dimensions are crucial for assessing the feasibility of vaginal birth and vary among women. It forms an angle of 60° with the horizontal. Its anterior arc is regular, while its posterior arc is deformed by the promontory. Its limits from anterior to posterior are the superior border of the pubic symphysis, pectineal crests, iliopectineal eminences, arcuate lines, and finally, the promontory.
The superior strait has anteroposterior, transverse, and oblique diameters. The anteroposterior diameters include the promontosuprapubic (minimum 11 cm, not the useful diameter) and the promontoretropubic (PRP) or useful anteroposterior diameter (minimum 10.5 cm). The transverse diameters include the maximum transverse (13.5 cm), the anterior transverse (12 cm, connecting the two iliopectineal eminences), and the median transverse or useful transverse (maximum 12 cm). The oblique diameters include the left oblique (12-13 cm, from the left iliopectineal eminence to the right sacroiliac articulation) and the right oblique (12 cm, from the right iliopectineal eminence to the left sacroiliac articulation). The left oblique diameter is the most frequently used by the fetus.
The pelvic excavation is located between the superior and inferior straits. It is the middle part where the descent and rotation of the fetus occur. Its limits include the posterior face of the pubic symphysis anteriorly, the anterior face of the sacrum posteriorly, and the quadrilateral faces, obturator foramen, and sacro-sciatic notches laterally. The internal face of the D3 moyen is marked by the ischial spines. All internal diameters of this cylinder-like excavation are 12 cm, except for the bispinous or bis-sciatic diameter (between the two ischial spines), which measures 10.8 cm.
The inferior strait is where the fetal exit occurs during the third stage of labor. It is delimited by four bone landmarks arranged in a rhombus: the inferior border of the pubic symphysis anteriorly, the ischial tuberosities laterally, and the tip of the coccyx posteriorly. The coccyx undergoes retropulsion (pushing backward) during delivery to create more space. The transverse diameter, the bis-ischial diameter (connecting the two ischial tuberosities), measures 11 cm and is undeformable. The anteroposterior diameter, the subcoccygeal-subpubic diameter, measures an average of 9.5 cm. While initially insufficient for delivery, it can extend to 12 cm due to coccyx retropulsion. The pubic arch (ogive pubienne), formed by the pubic symphysis and ischial branches, measures 85° to 90°.