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
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.
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
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.
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
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.
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
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.
2) Radiological estimation -> not used.
3) Sonographic Measurements -> BPD &
HC
- ?freely floating fetal head -> invalidate
the measurement.
- ?dolichocephlic (elongated in the occipito
frontal diameter)
?? -> underestimate fetal weight
& gestational age
?? -> HC measurement is more
accurate.
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
outlet.
- ?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
head.
¨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
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.
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
utility
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).