What are the key anatomical structures involved in pelvis anatomy during labour?

Key Anatomical Structures of the Pelvis Involved in Labour

The most important pelvic anatomical structures involved in labour include the bony pelvis, pelvic floor muscles (particularly the levator ani), and supporting ligaments and fascia that together form a functional unit critical for successful childbirth.

Bony Pelvis Structure

• The pelvic ring forms a closed compartment containing urogenital organs, rectum, vessels, and nerves, creating the rigid framework through which the fetus must pass during delivery 1, 2
• The sacrum, located at the posterior aspect of the pelvis, serves as a key bony component that articulates with the iliac bones to form the sacroiliac joints 2
• The pubic bones anteriorly form the pubic symphysis, which undergoes hormonal softening during pregnancy to allow slight movement during delivery 1
• Abnormal structure of vertebrae and pelvic bones can significantly complicate delivery in women with skeletal dysplasia 2

Pelvic Floor Musculature

• The levator ani muscle with its two primary bundles (pubococcygeus and iliococcygeus) forms the major component of the pelvic floor 3
• The levator ani is composed primarily of type I muscle fibers with high oxidative capability, making it function as a postural muscle that supports pelvic organs but also susceptible to injury during childbirth 3
• The levator hiatus is a critical opening in the pelvic floor through which the urethra, vagina, and rectum pass, and which must stretch significantly during vaginal delivery 1, 4
• Innervation of the pelvic floor comes primarily from the 2nd, 3rd, and 4th anterior sacral roots; denervation during childbirth can affect pelvic dynamics and contribute to dysfunction 3

Supporting Structures and Fascia

• The pelvic fascia covers the muscles and presents important reinforcements including the uterosacral and cardinal ligaments 3
• The arcus tendineus fascia pelvis (ATFP) and arcus tendineus levator ani (ATLA) are fascial reinforcements that provide attachment points for pelvic floor muscles 3
• The perineal body serves as an important anchoring point for multiple muscles at the posterior aspect of the perineum 3, 5

Functional Anatomy During Labour

• During labour, the angles formed by the pelvic viscera with their evacuation ducts undergo significant modification, expressing the action of musculofascial structures 3
• The pelvic floor must undergo substantial stretching to accommodate the fetal head, with the levator hiatus expanding significantly 6
• The support of pelvic viscera is multistructural, requiring intact pelvic floor muscles, proper organ topography, and an intact perineum 5
• The fetal head must navigate through the bony pelvis while the pelvic floor muscles stretch and thin to allow passage 6

Clinical Implications

• Vaginal childbirth can cause direct or denervation injury to the pelvic floor musculature, potentially leading to weakening of support structures 7
• Understanding the complex geometry of maternal pelvic floor muscles and fetal head motion during the second stage of labor remains challenging but critical for preventing birth-related injuries 6
• Pelvic floor muscle injuries during childbirth may contribute to later development of pelvic organ prolapse and urinary incontinence 7, 6
• Proper understanding of pelvic anatomy is essential for obstetric care providers to anticipate potential complications during labour 8, 6

Imaging Considerations

• MRI provides high-resolution imaging of pelvic floor musculature, allowing assessment of the levator ani muscle and detection of defects 1
• Dynamic pelvic floor MRI can demonstrate differences in pelvic floor musculature morphology and function 1
• Transperineal ultrasound can be used for real-time assessment of the pelvic floor during pregnancy and labour 1