CHAP 12, Mechanism of Normal Labor in Occiput Presentation
April 12, 1999
1st year resident. Jae-Hong Noh M.D.
Introduction
Fetus is in the occiput or vertex presentation in approximately 95%
Diagnosis of Occiput Presentation
1. Occiput Transverse Positions.
LOT position: 40%
ROT position: 20%
Finding of LOT position by abdominal examination.
First maneuver: Fundus occupied by breech
Second maneuver : back felt directly to the examiner's right
Third maneuver : fetal head is detected at or above the pelvic inlet
Fourth maneuver : cephalic prominence on the right side
On vaginal exam : saggittal suture occupies the transverse diameter of the pelvis more or less
midway between the sacrum and the symphysis
Fetal heart in right and left positions in usually heard in the right and left flank, respectively
2. Occiput Anterior Positions
Head enters the pelvis with the occiput rotated 45degrees anteriorly from the transverse position
This degree of anterior rotation produces only slight differences on abdominal examination
Mechanism of labor usually is very similar to that in occiput positions
3. Occiput Posterior Positions
Incidence of occiput posterior positions when the fetus enter the pelvis is approximately 20%
ROP is slightly more common than LOP
By radiographic study: .OP positions are more often associated with a narrow forepelvis
More commonly seen in association with anterior placentation
In the early part of labor, because of imperfect flexion of the head, larger anterior fontanel lies at a
Lower level than in anterior positions and is felt more readily
Cardinal Movement of Labor in Occiput Presentation
Figure 12-1
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Engagement : biparietal diameter, the greatest transverse diameter of the fetal head, pass through the pelvic inlet
Asynclitism : - Not lie exactly midway between the symphysis and sacral promontory
- Sagittal suture frequently is deflected posteriorly toward the promontory of anteriorly toward the symphysis
- If sagittal suture lies close o the symphysis and posterior parietal bone will present : Posterior asyncrlitism
- Moderate degrees of asynclitism are the rule in normal labor, but if severe, may lead to CPD
Figure 12-2
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Descent : with the nulliparous women, engagement may occur before the onset of labor, and further descent may not follow
until onset of 2nd stage of labor
Flexion : shorter suboccipitobregmatic diameter is substituted for the longer occipitofrontal diameter
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Figure 12-3. Lever action producing flexion of the head ; conversion from
occipitofrontal to suboccipitobregmatic diameter typically reduces
the an-teroposterior diameter from nearly 12 to 9.5 cm.
Internal Rotation : - occiput gradually moves from its original positions anteriorly toward the symphysis pubis,
internal rotation is essential for the completion of labor, except when the fetus is unusually small
- Internal rotations associated with descent of the presenting part,
is usually not accomplished until the head reached the level of the spines and therefore is engaged
Extension : the base of the occiput into direct contact with inferior margin of the symphysis pubis
- because of the vulvar outlet is directed upward and forward, extension must occur
- Two forces : 1st exerted by uterus ,act more posteriorly
2nd supplied by pelvic floor and symphysis, act more anteriorly
→Resultant vector is in the direction of the vulvar opening, causing extension
Figure 12-6. Mechanism of labor for left occiput anterior position.
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External Rotation : occiput was originally directed toward
return of head to the oblique position is followed by completion of external
rotation to the transverse position, a movement that corresponds to rotation of the fetal body
Expulsion : Ant shoulder → post shoulder
4. Labor in Occiput Posterior Position
Rotate to the symphysis through 135 degrees
Incomplete rotation : 5-10%
- fetus is large
- poor contraction
- epidural analgesia
faulty flexion of head
If rotation is incomplete or not take place, transverse arrest or Persistent occiput posterior results
Figure 12-7 Mechanism of labor for right occiput posterior position, anterior rotation
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Chang es in Shape of Fetal Head
Caput Succedaneum
In prolonged labors before complete dilatation of the cervix, the potion of the fetal scalp immediately over
the cervical os becomes edematous
More commonly caput is formed when the head is in the lower portion of the birth canal and
frequently only after the resistance of a rigid vaginal outlet is encountered
Molding
Because the various bones of skull are not firmly united, movement may occur at the suture
In many cases, one parietal bone may overlap the other anterior parietal usually overlaps the posterior
These changes are of greatest importance in contacted pelvis
Diminution in biparietal and suboccipitobregmatic diameters of 0.5 to 1.0 cm or even more in prolonged labor