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

    그림



  • 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
    그림


  • 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


    그림

    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.

    그림


  • 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
    그림



    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