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.

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

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