Mechanisms of Labor
Step by step
The Maternal Pelvis
During childbirth the fetus has to pass through the maternal pelvis. At the pelvic inlet it has an oval shape with its transverse diameter (13 cm) wider than its anteroposterior diameter (11 cm).
The pelvic outlet is anteroposterior ovally shaped with a transverse diameter of 11 cm and an anteroposterior diameter of approximately 11,5cm.
At the pelvic inlet the orientation of the birth canal is cranial to caudal. While at the level of the pelvic outlet it is dorsal to ventral. Fetal descent through the pelvis is therefore via a 90 degree angle, shown here by the pelvic axis. An imaginary line trough the center of the pelvic inlet, cavity and outlet.