MMS: Pelvimetry & Perineal Muscles

  • By: Terri
  • Date: May 26, 2010
  • Time to read: 2 min.

This is a very short summary of some of the important points in the lecture. It is not a complete representation, because of the extensive of graphics and images to illustrate a point, some things will be left to be updated later.

Read from Dr. Nilesh’s lecture.

Anatomical position of bony pelvis


  • ASIS & Symphysis is on the same coronal plane
  • Tip of coccyx & upper margin of symphysis in the same horizontal plane
    • So INLET makes an angle of 60 degree with transverse plane
    • directed forward

Stages in delivery

  • STAGE 1
    • cervical dilatation
    • starts with onset of true labour pain and ends with full dilatation of the cervix
  • STAGE 2
    • expulsion of fetus
    • Stages
      • Descent
        • uterine contractions & retractions
        • unfolding of fetus
      • Engagement
        • head engages in the oblique/transverse diameter of the inlet
        • suboccopito-bregmatic diameter
          • transverse diameter of inlet
          • should be more than 9.5cm
          • if less cannot accomodate fetal head engagement will fail –> cephalo-pelvic disproportion
      • Increased flexion
      • Internal rotation
      • Extension
  • STAGE 3
    • expulsion of placenta & membranes

Observe the diameters of the pelvic inlet & outlet. What are the conjugates related to pelvic diameter?

  • Anatomical conjugate
    • anteroposterior conjugate diameter
    • extends from the upper margin of the pubic symphysis to the middle of the sacral promontory
  • Obstetrical conjugate
    • shortest diameter through which foetal head must pass in it’s course throught the inlet
    • measured from middle of back of pubic symphysis to the sacral promontory
  • Diagonal conjugate
    • anteroposterior diameter of inlet as measured par vaginum
    • inability to palpate the sacral promontory suggests that the conjugate diameter of the inlet is adequate for parturition
      • palpated means contracted pelvis
    • distance between the lower margin of pubic symphysis & sacral promontory
    • Subtraction of diagonal conjugate by 1.5cm gives approximate measurement of anatomical conjugate


  • Female pelvis inlet: oval
    • diameter longer
    • transverse widest
    • suprapubic angle: 90 degree
    • Accomodate gap between thumb & index
    • greater sciatic notch: wider
    • pelvic cavity: long, conical
    • sacrum: short wide
  • Male pelvis inlet: heart shaped
    • subpubic angle: acute angle
    • accomodate gap between index & middle finger
    • greater sciatic notch: narrow fish-hook appearance
    • pelvic cavity: short, cylindrical
    • sacrum: long narrow

Classification of shape of pelvic inlet

  • Gynaecoid
    • rounded
  • Android
    • heart shaped (male)
  • Anthropoid
    • long, narrow, oval
  • Platypelloid (flat)
    • avoid, long axis tranverse
    • like a flat bowl

Supports of the uterus

  • Pelvic muscles
  • Ligaments

Muscles forming perineal body


Perineal body maintains integrity and should never be torn. If torn, can cause uterus/rectal prolapse. These muscles are supplied by the pudendal nerve.

  • levator ani (internal)
  • bulbospongiosus (external)
  • sphincter ani externus
  • transverse perineal
    • deep & superficial
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