[Obstetrical mechanics. Intermediate stage and childbirth stage].

Area of Science:

• Obstetrics and Gynecology
• Maternal-Fetal Medicine
• Biomechanics of Labor

Context:

• The second stage of labor, or expulsion stage, is a critical period in childbirth.
• Understanding the biomechanical events of this stage is crucial for optimizing delivery outcomes.
• Previous models may not fully capture the distinct anatomical and functional phases within the second stage.

Purpose:

• To delineate the second stage of labor into two distinct anatomical and functional phases.
• To elucidate the biomechanical roles of the sacrum and utero-sacral ligaments during labor.
• To explain the etiology of low backache in late pregnancy and labor and its relief.

Summary:

• The second stage of labor is divided into an intermediate phase (cervical dilation near complete, strong expulsive reflex initiating sacral oscillation/engagement) and a delivery phase (requiring sacral sliding/rotation and perineal relaxation).
• The utero-sacral ligaments, acting as tendons for the gravid uterus, participate in sacral oscillation.
• Low backache during late pregnancy and labor is attributed to utero-sacral ligament tension, exacerbated by difficult fetal head engagement, and is relieved by anterior pelvic tilting.

Impact:

• Provides a refined understanding of the biomechanics of labor, potentially informing clinical practices.
• Offers insights into the management of low back pain during pregnancy and labor.
• Enhances knowledge of the anatomical contributions to the expulsion stage of childbirth.