In passing the pelvis, the fœtus not only follows the curved line of the pelvic axis, but it describes a certain series of movements which alter its relations to the pelvis.
Fig. 44.—The child in left-occipito-anterior position. (Lenoir and Tarnier.)
There are five of these movements: flexion, descent, internal anterior rotation, extension, and external restitution.
Flexion.—Flexion is usually present before labor begins. That is, the head is bent down until the chin touches the breast. This may be modified by various conditions, but so far as it becomes extended, the mechanism is disturbed and the labor complicated, since large and less favorable diameters are brought to delivery.
Fig. 45.—The child in right-occipito-anterior position. Shows the flexion of the head intensified at the beginning of labor. (Eden.)
Flexion is increased by pressure against the pelvic brim as labor begins.
Descent.—As the driving force of the contractions becomes effective, the head passes the inlet and descends to the pelvic floor. When the large diameters of the head (biparietal) have passed the inlet, the head is said to be engaged.
