Dystocia 2

• OB2 ● OBSTETRICS 2
• Dystocia 2
• TRANS 2
• MODULE 1
• Presented by Jenny Lynn E. Idea, M.D., FPOGS, FPSUOG
• Lecture Outline

  July 24, 2024
• Abnormalities in Passage

  • Pelvic Capacity
  • The Pelvic Inlet
  • The Pelvic Midplane
  • The Pelvic Outlet
  • Clinical Pelvimetry
  • Pelvic Fractures
  • Imaging Studies of Bony Pelvis for Estimation of Pelvic Capacity
  • Soft Tissue Dystocia
  • Uterine Abnormalities
• Fetopelvic disproportion arises from diminished pelvic capacity or from abnormal fetal size, structure, presentation, or position.
• Pelvic Capacity Factors

  • Pelvic Inlet
  • Pelvic Midplane
  • Pelvic Outlet
  • Combination
• Pelvic Capacity


  Any contraction of the pelvic diameter that diminishes pelvic capacity can create dystocia.
• 3 AP Diameters

  • TRUE CONJUGATE
  • OBSTETRIC CONJUGATE
  • DIAGONAL CONJUGATE
• TRUE CONJUGATE

  DC - 1 cm ~ 11 cm
• OBSTETRIC CONJUGATE

  DC - 1.5 less than DC ~ 10 cm
• DIAGONAL CONJUGATE

  DC ~ 12 cm
• The Pelvic Inlet is CONTRACTED if Shortest AP diameter <10 cm, Transverse diameter <12 cm, Diagonal conjugate is <11.5 cm, No fetal head engagement.
• Vaginal exam to determine diagonal conjugate.
• The fetal biparietal diameter (BPD) averages from 9.5 to 9.8 cm.
• Fetal biparietal diameter (BPD) is less than pelvic inlet diameter


  It would prove difficult for a fetus to pass through the inlet.
• Patients with inlet contractions are more likely to have early spontaneous rupture of membranes.
• This is because the head is arrested at the inlet.
• The force of uterine contraction would be exerted directly to the portion of the membranes that is in contact with the dilated cervix.
• Once there’s early spontaneous rupture of membranes with contracted inlet, there’s no forward/hydrostatic pressure of the fetal head on the cervix & lower uterine segment.
• Less effective uterine contractions occur with early spontaneous rupture of membranes.
• Protracted or arrest in cervical dilatation occurs with early spontaneous rupture of membranes.
• Abnormal presentations with inlet contraction

  • Face or shoulder presentation
  • Cord prolapse
• Inlet contraction also leads to abnormal presentations with increased chance for face or shoulder presentation (3x more) and cord prolapse (6x more).
• The average midpelvis measures 10.5 cm in its transverse or interspinous diameter.
• Midplane Adequate Diameters

  • AP ≥ 11.5 cm
  • Transverse (IS) ≥ 10 cm
  • Posterior Sagittal ~5 cm
• Midplane Contracted Diameters

  • Interspinous < 8 cm
  • Midpelvic contraction is suspected whenever interspinous diameter is < 10 cm.
• Midplane contraction is more common than inlet contraction.
• Midplane contraction frequently causes transverse arrest of the fetal head which often leads to difficult mid-forceps extraction or cesarean section.
• Contracted pelvic outlet is defined as an inter-ischial-tuberous diameter of ≤ 8cm.
• Outlet Adequate Diameters

  • AP = 9.9 to 11.5cm
  • Intertuberous Diameter = 11 cm
• Outlet Contracted Diameters

  • Interischialtuberous Diameter ≤ 8cm
• Outlet contractions are insignificant in severe dystocia however, it may give rise to significant perineal tears.
• Clinical Pelvimetry Parameters

  • Adequate
  • Contracted
• Most common cause of pelvic fracture in pregnancy: trauma from vehicular accidents.
• Pelvic fracture during pregnancy is not an absolute indication of cesarean delivery.
• Once fracture has healed after 8-12 weeks, vaginal delivery may be considered, however, it needs radioimaging and pelvimetry later in pregnancy.
• Types of Imaging Pelvimetry

  • X-Ray
  • Computed Tomography
  • Magnetic Resonance Imaging