Birthing Pelvis with 2 Babies (Male and Female)

The birthing pelvis is the female bony structure essential for childbirth, providing both structural support and a dynamic passageway for fetal delivery.

Anatomical Composition

• Comprises four fused bones:

  • Paired iliac bones (forming the pelvic girdle)

  • Sacrum (posterior base of the spine)

  • Coccyx (tailbone)

• Connects the spine to the lower limbs, ensuring stability and weight distribution.

Shape & Structural Divisions

• Funnel/hourglass shape with distinct regions:

  • Upper flare (false pelvis): Wider, supports abdominal organs.

  • Lower flare (true pelvis): Narrower, forms the birth canal.

• Superior strait: The narrowing between these regions, a critical landmark for obstetric assessment.

Obstetric Straits & Key Measurements

The pelvis features three key straits, each with critical diameters for fetal passage:

1. Upper strait:

   • Promontoretropubic diameter (anteroposterior measurement).

2. Middle strait:

   • Median transverse diameter (widest point between ischial spines).

3. Lower strait:

   • Interspinous diameter (between ischial tuberosities).

• These measurements determine cephalopelvic disproportion (CPD) and delivery feasibility.

Dynamic Adaptability

• Not rigid: Hormonal changes (relaxin, progesterone) increase ligament laxity late in pregnancy.

• Pubis symphysis and sacroiliac joints slightly widen, expanding the birth canal by up to 10–15%.

Role in Childbirth

1. Guides fetal descent: The baby navigates through the pelvic inlet, mid-cavity, and outlet.

2. Facilitates rotation: Bone contours help the fetus rotate into optimal positioning (e.g., occiput anterior).

3. Enables vaginal delivery: Optimal pelvic dimensions and adaptability prevent obstructed labor.