OBGYN: Mechanism of Labor

Mechanism of Labor

- Series of changes in the attitude and position of the presenting part of the baby during its passage through the birth canal.

Attitude: relationship of parts of the baby to one another.

Position: Relationship between the fixed part of the maternal pelvis and the fixed part of the baby.

Denominator: Baby’s fixed part (varies with presentation).

Example: the chin is the denominator when the presentation is the face, sacrum is the denominator when the presentation is breech and occiput is the denominator when there is vertex presentation.

Note: Iliopectineal line is taken as an ANTERIOR point.

Sacroiliac joint is taken as a POSTERIOR point.

Pelvic Inlet:

- AP diameter = 11.5cm

- Transverse diameter = 13.5cm

- Therefore, Trasverse diameter > AP diameter (at the pelvic inlet)

- And so the baby enters the pelvis in the trasverse position

Pelvic Outlet:

- AP diameter = 13.5cm

- Transverse diameter = 11.5 cm

- AP diameter > Transverse diameter (at the pelvic outlet)

- The baby is delivered in the AP position

Steps of the Mechanism of Labor:

1. Descent and engagement
2. Flexion
3. Internal Rotation
4. Extension
5. Restitution
6. External Rotation

= Delivery of the baby!!!