Dystocia Due to Pelvic Contraction

Chapter 19 Dystocia Due to Pelvic Contraction

1st year resident, Lee Sun Joo

I. Contracted Pelvic Inlet

1. definition

  • ?shortest AP diameter < 10.0 cm or
    greatest transverse diameter < 12.0 cm.

  • ?AP diameter : commonly approximated
    by manually measuring the diagonal conjugate, which is about 1.5cm greater.

  • ?Inlet contraction : diagonal conjugate
    < 11.5cm.

  • ?Prior to labor, the fetal BPD has been
    shown to average from 9.5 to as much as 9.8cm in different populations.

  • ?The configuration of the pelvic inlet
    is also an important determinant of the adequacy of any pelvis.

  • ?A small woman : have a small pelvis
    & small infant.

  • ?In most species maternal size rather
    than paternal size is the important of fetal size.

2. Fetal Presentation and Position
  • ?A contracted inlet plays an important
    part in the production of abnormal presentations.

  • ?Contracted inlet -> descent usually
    does not take place until after the onset of labor, if at all.

  • ?In women with contracted pelves :
    • face & shoulder presentations -> 3 times.
    • cord prolapse -> 4 to 6 times more frequently.

3. Course of Labor.
  • ?Severe pelvic deformity -> labor is
    prolonged & effective spontaneous labor is often never achieved.

4. Maternal Effects

1) Abnormalities in Cervical Dilatation

  • ?When the head is arrested in the pelvic
    inlet the entire force acts directly upon the portion of membranes that
    overlie the dilating cervix.? ->?? Early ROM is more likely
    to result.

  • ?After membrane rupture, further dilatation
    may proceed very slowly or not at all.

  • ?Cervical response to labor provides
    a prognostic view of the outcome of labor.

2) Uterine rupture
  • ?CPD is so pronounced that there is no
    engagement & descent -> the lower uterine segmetn rupture may follow.

  • ?Pathological retraction ring -> immediate
    abdominal delivery is indicated.

3) Fistula Formation
  • ?Pelvic wall -> because of impaired circulation,
    necrosis may result -> vesicovaginal, vesicocervical, rectovaginal fistulas.

  • ?Pressure necrosis follows a very prolonged
    2nd stage of labor.

4) Intrapartum infection
  • ?by prolonged membrane rupture

  • ?by repeated cervical examinations &
    intrauterine manipulations.

5. Fetal Effects
  • ?contracted pelvis -> prolonged ROM &
    intrauterine infection? -> fetal & maternal risks are compounded.

1) Caput Succedaneum
  • ?contracted pelvis -> a large caput succedaneum
    develops on the most dependent part of the fetal head.

  • ?may assume considerable size & lead
    to serious diagnostic errors.

2) Fetal head molding
  • ?molding -> cranial plates overlapping
    one another at the major sutures.

  • ?frequently accomplished without obvious

  • ?may lead to tentorial tears, laceration
    of fetal blood vessels and fatal intracranial hemorrhage.

  • ?molding was greatest in the suboccipito
    bregmatic dimension & averaged 0.3cm with a range up to 1.5cm(The BPD
    was not affected).

  • ?Factors associated with molding

    • ?nulliparity
    • ?oxytocin labor stimulation
    • ?delivery with a vacuum extractor

  • ?locking mechanism -> protection for
    the fetal brain.

  • ?characteristic pressure marks?
    -> covering the portion of the head that passes over the promontory of
    the sacrum.

  • ?skull fractures -> usually following
    forcible attempts at delivery may occur with spontaneous delivery or even
    with cesarean delivery.

3) Umbilical Cord Prolapse
  • ?facilitated by imperfect adaptation
    between the presenting part and the pelvic inlet.

    ¨c rapid filling of the urinary
    bladder with 500 to 700ml of normal saline.

    ¨e intravenous ritodrine prior
    to cesarean delivery.

6. Prognosis
  • ?AP diameter of less than 9cm.?
    -> successful vaginal delivery of a term-sized fetus is nearly hopeless.

  • ?slightly below 10cm -> the prognosis
    for vaginal delivery.

    ¨c presentation : all presentations
    but the occiput are unfavorable.

    ¨e fetal size is of obvious

    ¨e pelvic inlet diameters &

    ¨e The frequency and intensity
    of uterine contractions are informative.

    ??? --> uterine dysfunction
    is common with significant disproportion.

    ¨e cervical response to labor.

    ¨i Extreme asynclitism &
    appreciable molding of the fetal head without engagement.

    ¨i Previous labor and delivery
    outcomes at term previous infant weights.

    ¨i Coincidental conditions that
    impair uteroplacental perfusion.

7. Management
  • ?A delivery that is safe for both mother
    and child cannot be anticipated.

  • ?Inlet contractions : weak uterine contractions
    during 1st-stage labor and a need for vigorous voluntary expulsive efforts
    during the 2nd stage.

II. Contracted Midpelvis

1. definition

  • ?midpelvis : inferior margin of the symphysis
    pubis? ¡­ ischial spines ¡­ sacrum near the junction
    of the 4th & 5th vertebrae.

  • ?A transverse line (connecting the ischial
    spines)?? -> divides the midpelvis into anterior & posterior

  • ?Anterior : lower border of the symphysis
    pubis ¡­ ischiopubic rami.

    ?Posterior : sacrum ¡­
    sacrospinous ligaments.

  • Average midpelvis measurements

    • transverse (interspinous) - 10.5 cm

    • anteroposterior??????????
      - 11.5 cm

    • posterior sagittal????????
      -? 5 cm

    (from the midpoint of the interspinous line to the same point on the sacrum)

  • Contracted midpelvis

    • interischial spinous + posterior sagittal
      diameters < 13.5 cm.

    • interischial spinous diameter < 8 cm.

2. identification
  • ?the spines are prominent.

  • ?the pelvic side walls converge.

  • ?the sacrosciatic notch is narrow.

3. prognosis
  • ?more common than inlet contraction.

  • ?a cause of transverse arrest of the
    fetal head.

4. management

  • ?perineum is bulging & vertex is
    visible -> the head has passed the obstruction.

  • ?strong fundal pressure should not be

  • ?vacuum extractor -> be of advantage
    after the cervix had become fully dilate

    ? -> should not be applied unless
    the BPD has passed the??? pelvic obstruction.

III. Contracted Pelvic Outlet

1. definition & incidence

  • ?diminution of the interischial tuberous
    diameter < 8cm.

  • ?anterior triangle : pubic rami. ¡­
    inferior posterior surface of the symphysis pubis.

    ?posterior triangle : tip of the
    last sacral vertebra.

2. prognosis

  • ?delivery -> partly depends on the size
    of the posterior triangle, or more on the interischial tuberous diameter
    and the posterior sagittal diameter of the outlet.

  • ?outlet contraction with concomittant
    midplane contraction.

  • ?the disproportion between the fetal
    head & the pelvic outlet

    ?-> the production of perineal tears.

IV. Pelvic Fractures and Pregnancy


  • ?careful review of previous X-rays &
    possibly CT pelvimetry later in pregnancy.

V. Rare Pelvic Contractions.


  • ?dwarfs, poliomyelitis, Kyphoscoliosis,
    small & dysmorphic women

    ?? -> cesarean delivery.

VI. Estimating Fetal Head Size & Pelvic Capacity.

1. fetal head size

1) clinical estimation

  • ?fundal pressure -> If no disproportion
    exists, the head readily enters the pelvis? -> vaginal delivery.

  • ?A flexed fetal head that overrides the
    symphysis pubis -> presumptive evidence of disproportion.

  • ?No relation between dystocia & failure
    of descent of the head.

2) Radiological estimation -> not used.

3) Sonographic Measurements -> BPD &

  • ?freely floating fetal head -> invalidate
    the measurement.

  • ?dolichocephlic (elongated in the occipito
    frontal diameter)

    ?? -> underestimate fetal weight
    & gestational age

    ?? -> HC measurement is more

2. Estimation of Pelvic Capacity

1) clinical estimation

  • Anteroposterior diameter of the inlet.

  • ?the interspinous diameter of the midpelvis

  • ?the intertuberous distances of the pelvic

  • ?A narrow pelvic arch(< 90 degrees
    ) -> narrow pelvis.

2) X-ray pelvimetry
  • ?5 factors of successful vaginal delivery.

    ¨c size & shape of the
    bony pelvis.

    ¨e size of the fetal head.

    ¨e force of the uterine contractions.

    ¨e moldability of the fetal

    ¨e presentation & position
    of the fetus.

  • ?Cephalic fetal presentation -> not necessary

        -> X-ray pelvimetry still is used(in breech).

3) Indications for X-ray Pelvimetry
  • ?The women with a previous injury or
    disease likely to affect the bony pelvis.

  • ?essential questions

    • ?1st : to affect the subsequent management
      of labor & delivery.

    • ?2nd : are other types of imaging techniques
      available for pelvimetry measurements.

4) Hazards of Diagnostic Radiation
  • ?The possibility of childhood malignancy
    was raised.

  • ?increased morbidity & martality
    have not been identified.

5) CT scanning
  • ?2 digital radiographs (AP & lateral

  • ? -> sufficient to measure the necessary
    pelvic diameters including the? interspinous.

  • ?fetal exposure :

    • ?maternal tissue thickness.

    • ?fetal size & position,

    • ?distance from the radiation source.

    • ?time of exposure.

6) Technique
  • ?AP view -> Electronic calipers are used
    to measure the transverse diameter of the inlet.

  • ?Lateral view -> AP diameter of the inlet
    & mid-pelvis.

  • ?At the center of the table.

  • ?Maternal movement is kept at a minimum.

  • ?Interspinous diameter -> an axial section
    through the fovea of the temoral heads.

7) Advantages
  • ?reduction in radiation exposure.

  • ?the greater accuracy.

  • ?easier to perform.

8) Ultrasound : without immediate clinical

9) MRI
  • ?lack of ionizing radiation

  • ?accurate pelvic measurements.

  • ?complete fetal imaging.

  • ?the potential for evaluating reasons
    for soft tissue dystocia.

  • ?expense, time, availability of equipment.

10) Combined Ultrasound & X-ray Pelvimetry
  • ?Fetal-pelvic index

    • ?fetal head & AC using ultrasound.

    • ?maternal pelvic inlet & midpelvic
      circumferences using X-ray pelvimetry.

  • ?Maternal pelvic dimensions & fetal
    AC were related to cesarean deliveries for fetopelvic disproportion

    ?(fetal HC was not identified as
    a cause of dystocia).