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Maternal pelvis

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Maternal pelvis

The maternal pelvis is the bony passage through which the fetus passes during vaginal birth. It consists of two hip bones, the sacrum, and coccyx.
Internal pelvic measurements and pelvic inlet

Divisions

  1. False (greater) pelvis
    • Above the pelvic brim
    • Supports the enlarging uterus in pregnancy
    • Not part of the bony birth canal
  2. True (lesser) pelvis
    • Below the pelvic brim
    • Forms the bony birth canal
    • Has three clinically important planes: inlet, cavity/midpelvis, and outlet

1. Pelvic inlet

The inlet is bounded by:
  • Posteriorly: sacral promontory
  • Laterally: linea terminalis
  • Anteriorly: upper border of symphysis pubis
Important diameters
  • Obstetric conjugate: shortest anteroposterior diameter, from sacral promontory to the posterior surface of the symphysis pubis. About 11 cm.
  • Diagonal conjugate: sacral promontory to inferior border of symphysis pubis. About 12.5-13 cm and can be estimated clinically.
  • Transverse diameter: about 13 cm.
Engagement occurs when the fetal biparietal diameter has passed the pelvic inlet. General Anatomy and Musculoskeletal System (THIEME Atlas of Anatomy), p. 161.

2. Midpelvis or pelvic cavity

  • Extends from the inlet to the level of the ischial spines.
  • The ischial spines are key obstetric landmarks.
  • The interspinous diameter is the narrowest transverse diameter of the pelvis, usually about 10-11 cm.
  • Fetal station is assessed in relation to the ischial spines:
    • Station 0: at the spines
    • Negative stations: above
    • Positive stations: below

3. Pelvic outlet

Boundaries:
  • Anteriorly: lower border of symphysis pubis and pubic arch
  • Laterally: ischiopubic rami and ischial tuberosities
  • Posteriorly: sacrotuberous ligaments and tip of coccyx
The anteroposterior outlet diameter is approximately 9 cm, but can increase by about 2 cm as the coccyx moves backward during birth. General Anatomy and Musculoskeletal System (THIEME Atlas of Anatomy), p. 161.

Pelvic types

TypeFeaturesObstetric relevance
GynecoidRounded inlet, wide subpubic arch, non-prominent ischial spinesMost favorable for vaginal birth
AndroidHeart-shaped inlet, narrow anterior pelvisMay cause deep transverse arrest or persistent occipitoposterior position
AnthropoidOval inlet with long AP diameterOccipitoposterior position is more common
PlatypelloidBroad transverse inlet and short AP diameterEngagement may be difficult

Obstetric importance

The fetal head must negotiate changing pelvic dimensions: the inlet is relatively widest transversely, while the outlet has a relatively greater anteroposterior dimension. This contributes to the fetal head’s internal rotation during normal labor. The pelvis is also not completely rigid: the symphysis pubis, sacroiliac joints, and sacrococcygeal joint gain some mobility in pregnancy under hormonal influence. Berek & Novak’s Gynecology, p. 150.
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