Fetal Orientation
&
Mechanism of Labor
Ob & Gy Department, First Hospital,
Xi’an Jiao Tong University
SHU WANG
Fetal Orientation
Fetal
lie

Fetal presentation

Fetal position
Fetal lie
• Refers to the relationship of the long axis of the fetus to
the long axis of the mother
• Longitudinal: 99% of all fetuses assume this lie.
• Transverse: long axis of the fetus is perpendicular to the long
axis of the mother.
• Result of over-relaxation of maternal abdominal wall
• Increased risk with multiple pregnancies
• Requires a C-section
• Oblique: the relationship of the spines forms <45 degree
angle.
Fetal lie
Longitudinal
Transverse
Oblique
Fetal presentation
• Presenting part
Refers to the part of the fetus that enters the canal first
• Cephalic
• Vertex: head with chin flexed toward chest (most common cephalic
position)
• Face: fetus’s neck is hyperextended bringing the back of its head
• Breech
3% presentations
• Shoulder
• Compound
Cephalic presentation
Vertex
presentation
face
presentation
Breech presentation
•Frank breech (50-70%) - Hips flexed, knees extended (pike position)
•Complete breech (5-10%) - Hips flexed, knees flexed (cannonball)
•Footling or incomplete (10-30%) - One or both hips extended, foot
presenting
Shoulder
presentation
Compound
presentation
Normal presentation
Abnormal presentations
Fetal position
• Determines the relationship of a designated point
of the presenting part in relation to the maternal
pelvis
Presentation
Point of reference
Vertex
Occiput
Face
Chin (Mentum)
Breech
Sacrum
Shoulder
Scapula
Fetal position
• Designated points used for the maternal pelvis follow:
Left Occipital Anterior
Right Occipital Anterior
Anterior
Right
Left
posterior
Right Occipital Posterior
Left Occipital Posterior
–LOA: fetal head is presenting with occiput directed toward the
left side and anterior pelvis of the woman
Occiput position
Right
Left Occiput Anterior
LOP
Transverse
Left Occiput Transverse
LOT
Left
Left Occiput Posterior
LOP
Mentum
position
Left Mentum Anterior
LMA
Left Mentum Posterior
LMP
Mechanism of
Labor
The sequencing of events related to posturing
and positioning that allows the baby to find the
"easiest way out."
Mechanism of labor: a series of passive,
adaptive movements of the fetal head and
shoulders’ smallest diameter to pass through the
birth canal
Review: Pelvis
Pelvic inlet
A-P d 11cm
Pelvic outlet
Oblique d.12.75cm
Transverse d13cm
midpelvis
A-P d
A-P d.11.5cm
Transverse d10cm
Transverse d.9cm
Review: Axis
of Pelvis
Cranial
Dorsal
Ventral
Caudal
Review: Fetal cranium
Review: Fetal cranium
Occipitofrontal d. 11.3cm
Suboccipitobregmatic d. 9.3cm
Fetus makes his head smaller
flexion
Occipitofrontal diameter
>
Suboccipitobregmatic diameter
Mechanism of Labor
Positional movements the fetus undergoes to
accommodate itself through the maternal pelvis
Engagement
Birth of the shoulders
Descent
External rotation
Flexion
Restitution
Internal rotation
Extension
Engagement
Occurs when the fetal
presenting part has
passed through the
maternal inlet of the
pelvis
Descent
continuous downward
movement of fetus through
birth canal
1. pressure by the amniotic fluid
2. direct pressure by the contracting fundus on the fetus
3. force of the contraction of the maternal diaphragm and abdominal muscles in
second stage labor
4. extension and straightening of the fetal body
Flexion
•
shape of the uterus causes
fetal neck to flex and the
chin rests on its chest
Allows the smallest diameter of the head to pass through the smallest
portion of the maternal inlet
Internal rotation
•Allows fetal head
to pass beneath the
pelvic arch
rotation from a diagonal or
transverse position to an
anteroposterior position
Extension
• Fetal head
extends to allow
the birth of the
occiput, face, and
chin
• Occurs in
response to the
pressure from
uterine
contractions and
shape of pelvic
floor
Restitution and external rotation
• After head is delivered, it
immediately rotates to the left
or right
•Assumes previous position
that it was in during
engagement
•Anterior shoulder is
delivered first followed by the
posterior shoulder
Expulsion
Once
shoulders are
delivered, rest
of the body
just slides out
Critical
Movements
of labor
No two pelvis are exactly the same, just as no
two faces are the same. For each pelvis there is an
optimum mechanism that may be wholly different
from the so-called normal mechanism.
-------Caldwell and Maloy
The end!
Picture
• These children wish they could go home
to a nice family but they have no home
to go to – everyday is a misery.
Fantasies Can Come True
Let’s make these children’s fantasies a reality.