(measuring ear tip to ear tip across the top of the baby's head)
into the pelvic inlet.
Descent
The baby's head moves deep into the pelvic cavity and is
commonly called lightening. The baby's head becomes markedly
molded when these distances are closely the same. When the occiput is
at the level of the ischial spines, it can be assumed that the biparietal
diameter is engaged and then descends into the pelvic inlet.
Flexion
This movement occurs during descent and is brought about by the
resistance felt by the baby's head against the soft tissues of the pelvis.
The resistance brings about a flexion in the baby's head so that the chin
meets the chest. The smallest diameter of the baby's head (or
suboccipitobregmatic plane) presents into the pelvis.
Internal rotation
As the head reaches the pelvic floor, it typically rotates to accommodate
for the change in diameters of the pelvis. At the pelvic inlet, the diameter
of the pelvis is widest from right to left. At the pelvic outlet, the
diameter is widest from front to back. So the baby must move from a
sideways position to one where the sagittal suture is in the
anteroposterior diameter of the outlet (where the face of the baby is
against the back of the laboring woman and the back of the baby's head
is against the front of the pelvis). If anterior rotation does not occur, the
occiput (or head) rotates to the occipitoposterior position. The
ocipitoposterior position is also called persistent occipitoposterior and is the common cause for
true back labor.
Extension
After internal rotation is complete and the head passes through the pelvis at the nape of the neck,
a rest occurs as the neck is under the pubic arch. Extension occurs as the head, face and chin are
born.
External rotation
After the head of the baby is born, there is a slight pause in the action of labor. During this pause,
the baby must rotate so that his/her face moves from face-down to facing either of the laboring
woman's inner thighs. This movement, also called restitution, is necessary as the shoulders must
fit around and under the pubic arch.
It is at this point that shoulder dystocia may be identified. Shoulder dystocia occurs when the
baby's shoulders are halted at the pelvic outlet due to inadequate space through which to pass.
Mother's birthing babies who are identified as macrosomatic (in excess of 9.9 lbs.) are more
likely to experience sho ulder dystocia. Additionally, 15-30% of macrosomatic babies
experiencing shoulder dystocia sustain some injury to the brachial plexus. Most of these injuries
(80%) resolve by the baby's first birthday.
Commonly, the McRobert's technique is used to resolve shoulder dystocia. This technique
involves a sharp flexing of the maternal thighs against the maternal abdomen to reduce the angle
between the sacrum and the spine.
Expulsion
Almost immediately after external rotation, the anterior shoulder moves out from under the pubic
bone (or symphisis pubis). The perineum becomes distended by the posterior shoulder, which is
then also born. The rest of the baby's body is then born, with an upward motion of the baby's
body by the care provider.
