What are the key anatomical structures of the pelvis involved in labor?

Anatomy of the Pelvis Involved in Labour

The key anatomical structures of the pelvis involved in labor include the bony pelvis, pelvic floor muscles, and soft tissues that undergo significant biomechanical changes to facilitate vaginal delivery.

Bony Pelvis Structure

• The pelvic ring forms a closed compartment of bones containing urogenital organs, rectum, vessels, and nerves 1
• Key bony components include:
  • Iliac bones (paired)
  • Sacrum (posterior)
  • Coccyx (tailbone)
  • Pubic symphysis (anterior joint) 2
• During labor, the pelvic bones undergo subtle but demonstrable movements in all anatomical planes (sagittal, frontal, and transverse) to gradually expand the space in the lesser pelvis 2

Pelvic Joints and Mobility

• The pelvis contains several joints that allow movement during labor:
  • Pubic symphysis - can widen up to 3-6 mm during delivery 3
  • Sacroiliac joints - allow for sacral movement 2
  • Sacrococcygeal joint - permits coccyx rotation of up to 15.7° in flexible positions 3
• Two key movements occur during labor:
  • Counternutation of the sacrum - helps expand the plane of the pelvic inlet 2
  • Nutation of the sacrum - assists in expanding the pelvic width, height, and outlet 2

Pelvic Floor Muscles

• The levator ani muscle complex is the primary pelvic floor muscle group involved in labor 4
  • Includes pubococcygeus, iliococcygeus, and puborectalis muscles
  • Forms a hammock-like structure supporting pelvic organs 5
• The obturator internus muscles form part of the lateral pelvic walls 5
• During vaginal delivery, the levator ani muscles can stretch to an extraordinary ratio of 3.26 by the end of the second stage of labor 4
• MRI studies show that regions experiencing the most stretch are at greatest risk for injury, especially during forceps deliveries 4

Soft Tissues and Organs

• The vagina is suspended across the midline, attaching bilaterally to the obturator and levator ani muscles 5
• The vagina supports the bladder and urethra above it 5
• The rectum fits in a midline groove in the levator ani 5
• Soft tissues of the pelvic floor undergo significant stretching during labor, with the degree of elasticity affecting delivery forces 6

Biomechanical Changes During Labor

• Different birthing positions lead to significant changes in pelvic space availability 3:
  • Flexible sacrum positions (kneeling, standing, squatting, sitting) allow greater coccyx movement (15.7°) and less pubic symphysis widening (3 mm) 3
  • Non-flexible sacrum positions restrict coccyx movement (3.6°) and require greater pubic symphysis widening (6 mm) 3
• The birthing position significantly affects the biomechanics of delivery and can influence:
  • The widening of the pubic symphysis
  • The rotation of the coccyx
  • Stress distribution in pelvic floor muscles 3

Clinical Implications

• Abnormal structure of vertebrae and pelvic bones can complicate delivery in women with skeletal dysplasia 1
• Childbirth-related anatomic and neurological injury to the pelvic floor may be associated with pelvic floor dysfunction later in life 5
• Approximately 11% of U.S. women undergo surgery for pelvic floor dysfunction, including genital organ prolapse and urinary and fecal incontinence, with vaginal birth being the major risk factor 4
• Understanding pelvic biomechanics helps obstetricians support progress in cases of non-progressive labor by optimizing maternal positioning 2