Introduction to the Obstetric Pelvis
The bony pelvis is a sturdy, basin-shaped ring of bone that connects the spine to the lower limbs. For midwives practicing in Nigeria, understanding this structure is the foundation of managing labor and childbirth safely. The pelvis protects the pelvic organs, supports the weight of the pregnant uterus, and acts as the bony canal through which the baby must pass during vaginal delivery. It is structurally divided into the false pelvis above and the true pelvis below, with the true pelvis being the actual birth canal.
The Four Bones of the Pelvis
The bony pelvis is formed by four distinct bones that fuse firmly together or articulate closely by adulthood.
- Innominate bones: Also known as the hip bones, there are two of them (right and left). Each innominate bone is formed by the fusion of three parts: the ilium, ischium, and pubis.
- Sacrum: A wedge-shaped bone composed of five fused vertebrae located at the back of the pelvis between the two hip bones.
- Coccyx: The tailbone, consisting of four fused vertebrae attached to the lower tip of the sacrum, which can move backward during delivery to increase pelvic outlet space.
Pelvic Joints and Their Mobility in Pregnancy
The bones of the pelvis are joined together by strong articulations. Unlike rigid joints elsewhere, pregnancy hormones such as relaxin cause these joints to soften and increase in laxity.
- Pubic symphysis: The cartilaginous joint connecting the two pubic bones anteriorly. It widens slightly during pregnancy to accommodate the passing fetus.
- Sacroiliac joints: Strong, weight-bearing synovial joints linking the sacrum to the ilium on both sides.
- Sacrococcygeal joint: The articulation between the sacrum and the coccyx, allowing the coccyx to flex backward during second-stage labor.
Crucial Pelvic Ligaments
Several tough, fibrous bands stabilize the pelvic joints and prevent excessive movement, which is especially important given the softening effects of pregnancy hormones.
- Sacrotuberous ligament: Runs from the sacrum and coccyx to the ischial tuberosity, converting the lesser sciatic notch into a foramen.
- Sacrospinous ligament: Runs from the sacrum and coccyx to the ischial spine, crossing horizontally below the sacrotuberous ligament.
- Iliolumbar ligament: Connects the transverse process of the fifth lumbar vertebra to the iliac crest, stabilizing the lumbosacral junction.
Landmarks of the False and True Pelvis
An imaginary line called the pelvic brim (or linea terminalis) separates the pelvis into two distinct regions. Midwives must master these specific anatomical landmarks to assess pelvic adequacy.
- False pelvis: Situated above the pelvic brim. It supports the abdominal viscera and gravid uterus during pregnancy but has no direct obstetric significance in terms of birth dimensions.
- True pelvis: Situated below the pelvic brim. It contains the inlet, cavity, and outlet through which the baby must navigate. Key landmarks here include the sacral promontory at the back, the ilpsopectineal line on the sides, and the pubic crests in front.
Obstetric Significance and Clinical Implications
A thorough understanding of pelvic anatomy directly guides clinical midwifery practice. The shape and dimensions of the true pelvis determine whether a fetus can pass safely through the birth canal. Midwives use surface landmarks, such as the ischial spines, to assess the station of the presenting part during vaginal examinations. Variations in pelvic architecture—such as gynecoid, android, anthropoid, or platypelloid shapes—directly influence the mechanism of labor and the likelihood of successful spontaneous vaginal delivery.