Mechanism of Normal Labor in Occiput Presentation
CHAP 12, Mechanism of Normal Labor in Occiput Presentation
April 12, 1999
1st year resident. Jae-Hong Noh M.D.
April 12, 1999
1st year resident. Jae-Hong Noh M.D.
- 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
¡æResultant 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 ¡æ post shoulder
- LOT position: 40%
Introduction
- 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
4. Labor in Occiput Posterior Position