Signs of impending labor

Basic concepts and start of the process

The concept of the mechanism of labor describes the entire set of movements performed by the fetus while passing through the birth canal. The main biological and biomechanical goal of this process is for the fetal head to overcome the larger dimensions of the pelvis using its smallest dimensions.

The mechanism itself is triggered at a strictly defined moment: when the advancing head encounters an obstacle preventing its further rectilinear movement.

Biomechanics of fetal advancement

Movement of the fetus through the birth canal is ensured by powerful expulsive forces. Advancement occurs not chaotically, but strictly along the direction of the pelvic axis.

• The pelvic axis is an imaginary line connecting the midpoints of all straight dimensions of the pelvis.
• Shape of the axis resembles a fishhook.
• Such an arched trajectory is due to the anatomical structure: the natural curvature of the sacrum and the presence of a powerful layer of pelvic floor muscles.

Anatomical components of the birth canal

The structural features of the birth canal directly affect fetal advancement by offering resistance. Anatomically, they are divided into two main groups:

1. Soft tissues: include the lower uterine segment, vagina, fascia, muscles of the inner surface of the lesser pelvis, and perineum. As the fetus advances, they stretch, creating physical resistance.
2. Bony framework: the bony ring has varying widths at different levels. For an accurate obstetric assessment of fetal advancement and determination of the head station, it is customary to distinguish 4 main planes of the lesser pelvis:
3. Plane of the pelvic inlet;
4. Plane of the wide part of the pelvic cavity;
5. Plane of the narrow part of the pelvic cavity;
6. Plane of the pelvic outlet.

Features of the expulsion stage

During the expulsion of the fetus, the trajectory of the head's movement becomes clearly visible. It moves strictly along the arched pelvic axis, skirting the pubic symphysis and deflecting the coccyx. It is precisely these anatomical landmarks that determine the correct direction of biomechanical movements.

Class and exam questions

What else to study on this topic

• Detailed analysis of each of the four levels of the lesser pelvis planes
• Anatomical features of the structure of the sacrum and pelvic floor muscles
• Characteristics of the expulsive forces of the uterus and abdominal press
• Physiological changes in the soft tissues of the birth canal during stretching
• Detailed analysis of the expulsion period based on diagrams

Other topics in the "Obstetrics" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.