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Mechanics of childbirth

Last updated: April 21, 2021

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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.

Fetal lie

  • Definition: relation of the fetal long axis to the long axis of the maternal uterus
  • Types
    • Longitudinal lie: fetus is in the same axis (most common)
    • Transverse lie: fetus is at a 90° angle
    • Oblique lie: fetus is at a 45° angle

Fetal presentation

  • Definition: part of the fetus that overlies the maternal pelvic inlet
  • Types
    • Cephalic presentation: head (most common)
    • Breech presentation: buttocks or feet
      • Frank breech: flexed hips and extended knees (buttocks presenting)
      • Complete breech: thighs and legs flexed (cannonball position)
      • Single Footling breech: hip of one leg is flexed and the knee of the other is extended (one foot presenting)
      • Double Footling breech: both thighs and legs are extended (feet presenting)
    • Compound presentation: ≥ 1 anatomical presenting part (e.g., cephalic or breech presentation with presentation of an extremity)
    • Shoulder presentation: shoulder presentations combined with a transverse or oblique lie

Fetal position

  • Definition: relationship and orientation (i.e., fetal occiput pointing towards maternal left or right) of the presenting fetal part to the maternal pelvis
  • Types
    • Occiput anterior position: Fetal occiput points towards maternal symphysis pubis; fetus faces downwards.
      • Left occiput anterior (LOA): Fetal back faces the maternal left, anterior fontanelle faces the maternal right, sagittal suture lies in the right oblique diameter; (most common position).
      • Right occiput anterior (ROA): Fetal back faces the maternal right, anterior fontanelle faces the maternal left, sagittal suture lies in the left oblique diameter.
    • Occiput posterior position: Fetal occiput points towards the maternal sacral promontory with face to pubis symphysis; the fetus faces upward
    • Sacrum in breech presentation
    • Mentum (chin) in extended cephalic (face) presentation

Fetal attitude [1]

  • Definition: degree of extension/flexion of the fetal head during cephalic presentation
  • Types
    • Vertex presentation (maximally flexed); most common attitude
    • Brow presentation (partially extended)
    • Face presentation (maximally extended)
    • Forehead presentation (partially flexed; military attitude): Spontaneous vaginal delivery is possible .

Station (Obstetrics) [2]

  • 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

Synclitism

  • 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.
    • Anterior asynclitism (Naegele obliquity)
      • The sagittal suture is positioned towards the sacral promontory
      • Spontaneous vaginal delivery possible
    • Posterior asynclitism (Litzmann obliquity)

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

References:[1]

Overview [1]

Management [1]

  1. Dutta DC, Konar H. Textbook of Obstetrics. Jaypee Brothers Medical Publishers ; 2015
  2. Buchmann EJ, Guidozzi F. Level of fetal head above brim: comparison of three transabdominal methods of estimation, and interobserver agreement.. J Obstet Gynaecol. 2007; 27 (8): p.787-790. doi: 10.1080/01443610701667387 . | Open in Read by QxMD
  3. Dudenhausen JW, Obladen M. Practical Obstetrics. Walter de Gruyter GmbH & Co KG ; 2014
  4. Kaplan. USMLE Step 2 CK Lecture Notes 2017: Obstetrics/Gynecology . Kaplan Medical ; 2016