2 - Pelvis 2026

Pelvis

Bony ring composed of sacrum, coccyx, and two innominate bones

Innominate bone

Fusion of ilium, ischium, and pubis

Symphysis pubis

Fibrocartilaginous joint uniting pubic bones anteriorly

Linea terminalis

Divides false (greater) and true (lesser) pelvis

False (greater) pelvis

Portion above linea terminalis supporting abdominal organs: Boundaries: Anterior: lower portion of the anterior abdominal wall, Posterior: lumbar vertebra, Lateral: iliac fossa

True (lesser) pelvis

Portion below linea terminalis forming the birth canal

Planes of the true pelvis

Inlet, outlet, midpelvis, plane of greatest pelvic dimension

Boundaries of the pelvic inlet

Posterior: sacral promontory, Lateral: linea terminalis, Anterior: horizontal pubic rami, symphysis pubis

Diagonal conjugate

Lower margin of symphysis pubis to sacral promontory; measured vaginally

Obstetric (true) conjugate

Shortest AP inlet diameter; DC − 1.5-2 cm (≥10 cm)

Transverse diameter (inlet)

Widest distance between linea terminalis (~13 cm)

Oblique diameter (inlet)

Sacroiliac joint to contralateral iliopectineal eminence

Pelvic inlet - clinical significance

Site of fetal head engagement; obstetric conjugate is limiting

Landmark of the midpelvis

Level of ischial spines

Interspinous diameter

Distance between ischial spines; smallest pelvic diameter (≥10 cm)

Anteroposterior midpelvic diameter

Approximately ≥11.5 cm

Midpelvis - clinical significance

Most common site of labor arrest; defines fetal station

Boundaries of the pelvic outlet

Posterior: coccyx, Posterolateral: sacrotuberous ligaments, Anterior: Pubic symphysis, Anterolateral: ischiopubic rami, ischial tuberosities

Caldwell-Moloy classification

Pelvic types based on greatest transverse inlet diameter

Gynecoid pelvis

Round inlet; most favorable for vaginal delivery (~50%)

Android pelvis

Heart- or funnel-shaped inlet; associated with labor arrest

Anthropoid pelvis

Oval inlet (AP > transverse); ↑ occiput posterior positions

Platypelloid pelvis

Flat inlet with wide transverse diameter; least common, unfavorable

Mixed pelvis

Combination of pelvic types; common