• Clinical science

Mechanics of childbirth


The process of normal childbirth depends on a high degree of anatomical and physiological compatibility between the mother and child. The birth canal is the passage consisting of the mother's bony pelvis and soft tissues through which a fetus passes during vaginal delivery. Fetal orientation during childbirth is described in terms of lie, presenting part, position and attitude of the presenting part, and station. Complications of labor and delivery can pose life-threatening situations to both mother and child, which is why recognizing any abnormalities early is crucial. Measures for managing abnormal labor include special maneuvers for breech presentation or cesarean delivery in obstructed labor.

Orientation in utero

Fetal lie

Fetal presentation

Fetal position

Fetal attitude

Station (Obstetrics)

  • Definition: measurement (in cm) of the presenting part above and below the maternal ischial spine
    Station Description
    0 the presenting part is at the level of the ischial spines
    -1, -2, -3 1, 2, and 3 cm above the level of the ischial spines, respectively
    +1, +2, +3 1, 2, and 3 cm below the level of the ischial spines, respectively
  • Engagement (Obstetrics)
    • When the widest transverse diameter of the head (presenting part) passes through the pelvic inlet
    • Use the rule of fifths: engagement is clinically identified when ≤ 2/5 of the fetal head are felt above the symphysis pubis through the maternal abdomen


  • Definition: parallelism between the pelvic plane and the plane of the fetal head
  • In asynclitism, the sagittal suture is in the transverse diameter of the pelvic inlet and not between the symphysis pubis and sacral promontory.


Normal labor and delivery

Adaptation to the different forms of the pelvic region requires a great deal of rotation.

  1. Engagement, descent, and increased flexion (occur simultaneously)
    • The head engages below the plane of the pelvic inlet.
    • The presenting part begins to descend into the birth canal.
    • The chin of the fetus moves towards its chest.
  2. Internal rotation: The fetal head rotates by 90° (two 45° steps) in the midpelvis, from a transverse to anterior-posterior position.
  3. Extension: The fetal head, lying behind the symphysis pubis bone and the pelvic floor, acts upwards and forwards.
  4. Restitution: The fetal head rotates 45° in the opposite direction as it passes through the pelvic outlet.
  5. External rotation: The anterior shoulder rotates 45° anteriorly as it meets the maternal pelvic floor. This action is transmitted to the head which also rotates 45°, placing the head in its original transverse position.
  6. Expulsion: Delivery of the head, anterior shoulder followed by the posterior shoulder, and the body


Obstructed labor


Arrest of vaginal delivery because of a mechanical obstruction (see arrested active phase for comparison)


  • Fetal: malpresentations; , malpositions, congenital anomalies, macrosomia
  • Maternal: bony or soft tissue masses in maternal pelvis

Clinical features



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  • 6. Hofmeyr GJ, Lockwood CJ, Barss VA. External Cephalic Version. In: Post TW, ed. UpToDate. Waltham, MA: UpToDate. https://www.uptodate.com/contents/external-cephalic-version. Last updated July 18, 2017. Accessed September 6, 2017.
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last updated 11/18/2020
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