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.
  

• ?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.
    
  
  

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

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.
  

• ?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.
  

• ?Pelvic wall -> because of impaired circulation,
  necrosis may result -> vesicovaginal, vesicocervical, rectovaginal fistulas.
  
• ?Pressure necrosis follows a very prolonged
  2nd stage of labor.
  

• ?by prolonged membrane rupture
  
  
• ?by repeated cervical examinations &
  intrauterine manipulations.
  

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

• ?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.
  

• ?molding -> cranial plates overlapping
  one another at the major sutures.
  
  
• ?frequently accomplished without obvious
  detriment.
  
  
• ?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.
  

• ?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.

• ?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
  importance.
  
  ¨e pelvic inlet diameters &
  configuration.
  
  ¨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.

• ?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
  portions.
  
  
• ?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.
    
  
  

• ?the spines are prominent.
  
  
• ?the pelvic side walls converge.
  
  
• ?the sacrosciatic notch is narrow.
  

• ?more common than inlet contraction.
  
  
• ?a cause of transverse arrest of the
  fetal head.
  

• ?perineum is bulging & vertex is
  visible -> the head has passed the obstruction.
  
  
• ?strong fundal pressure should not be
  used.
  
  
• ?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.
  

• ?freely floating fetal head -> invalidate
  the measurement.
  
  
• ?dolichocephlic (elongated in the occipito
  frontal diameter)
  
  
  ?? -> underestimate fetal weight
  & gestational age
  
  ?? -> HC measurement is more
  accurate.

1) clinical estimation

• Anteroposterior diameter of the inlet.
  
  
• ?the interspinous diameter of the midpelvis
  
  
• ?the intertuberous distances of the pelvic
  outlet.
  
  
• ?A narrow pelvic arch(< 90 degrees
  ) -> narrow pelvis.
  

• ?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
  head.
  
  ¨e presentation & position
  of the fetus.
  
• ?Cephalic fetal presentation -> not necessary
  
  
  ???????????????????????????
  -> X-ray pelvimetry still is used(in breech).
  
  
  
  

• ?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.
    
  
  

• ?The possibility of childhood malignancy
  was raised.
  
  
• ?increased morbidity & martality
  have not been identified.
  

• ?2 digital radiographs (AP & lateral
  views)
  
  
• ? -> 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.
    
  
  

• ?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.
  

• ?reduction in radiation exposure.
  
  
• ?the greater accuracy.
  
  
• ?easier to perform.
  

• ?lack of ionizing radiation
  
  
• ?accurate pelvic measurements.
  
  
• ?complete fetal imaging.
  
  
• ?the potential for evaluating reasons
  for soft tissue dystocia.
  
  
• ?expense, time, availability of equipment.
  

• ?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).