The birth passage is made up of three parts, the pelvic inlet, pelvic cavity, and the pelvic outlet.
The bony pelvis can be classified into four types:
Among them, the gynecoid type of pelvis is the most commonly found and has the diameters that are structurally ideal for vaginal delivery and child birth.
- In the gynecoid pelvis, the anterior/posterior and lateral measurements in the inlet, midpelvis, and outlet of the true pelvis are largest.
- The inlet is laterally round to oval-shaped; it is a feature that enhances its capability for childbirth.
- The ischial spines are less protuberant compared to other types.
- A deep, wide curve is found in the shortened sacrum.
- The subpubic arch is wide as well as round.
- All these features improve the capability for a vaginal birth.
- At the time of labor it enables early and complete fetal internal rotation.
- The other pelvic structures may create problems for vaginal birth.
- In all three planes of the pelvis this shape has the best diameters.
The gynecoid pelvis is found in approximately 50% of all women and considered a true female pelvis; it is rare in men. Due to the round inlet and spacious pelvic outlet, vaginal birth is easier in this type of pelvis.