Pelvis from an obstetrical perspective

Bony framework and pelvic divisions

The adult female pelvis is formed by four bones: two hip bones (ossa coxae), the sacrum (os sacrum), and the coccyx (os coccygis). Each hip bone consists of three fused parts: the ilium, ischium, and pubis.

The bones are connected by slightly movable joints with cartilaginous interlayers: the pubic symphysis (symphysis), paired sacroiliac joints, and the sacrococcygeal symphysis.

Anatomically, the pelvis is divided into two parts:

• Greater (false) pelvis. Bounded by the iliac wings and lumbar vertebrae. There is no anterior bony wall. The fetus does not pass through it, but its dimensions allow the physician to indirectly estimate the internal measurements.
• Lesser (true) pelvis. This is the actual birth canal. It is a truncated cylinder where the anterior wall (symphysis) is almost three times shorter than the posterior wall (sacrum and coccyx).

Planes of the lesser pelvis and their dimensions

The space of the lesser pelvis is heterogeneous. To conveniently assess the advancement of the fetus, four imaginary planes are distinguished. Each has its own boundaries and obstetric dimensions.

Biomechanical landmarks: axis and angle of inclination

The fetus does not drop straight down, but performs complex rotations, following the curves of the birth canal.

• Pelvic axis (axis pelvis) is an imaginary arc connecting the centers of the anteroposterior diameters of all planes. It is concave anteriorly and convex posteriorly, resembling a fishhook. At the inlet plane, the axis is directed obliquely backward, at the narrow part — straight down, and at the outlet — forward.
• Angle of pelvic inclination (inclinatio pelvis) is the angle of intersection between the inlet plane and the horizontal line. In a standing pregnant woman, it normally measures 45–50°. This angle can be altered: if the woman lies on her back and tightly pulls her thighs to her abdomen (or squats), the angle decreases. If a roll is placed under the lower back, it increases.

Types of the female pelvis

According to the Caldwell-Moloy classification (1934), based on the shape of the pelvic inlet, four anatomical types are distinguished:

1. Gynecoid — the classical normal female type with a rounded or slightly transverse-oval inlet.
2. Android — male-type pelvis. The inlet narrows anteriorly, resembling a heart shape (like in playing cards).
3. Anthropoid — longitudinally oval inlet, where the anteroposterior diameter exceeds the transverse diameter.
4. Platypelloid — flat pelvis with a pronounced transverse-oval inlet shape (anteroposterior diameter shortened, transverse diameter increased).

Class and exam questions

What else to study on this topic

• Detailed anatomical structure of the ilium, ischium, and pubis
• Oblique diameters of the pelvic inlet plane (right and left)
• Anatomical conjugate and its difference from the true conjugate
• Exact bony boundaries of each of the four planes of the lesser pelvis
• Influence of the parturient woman's posture on labor biomechanics and the pelvic inclination angle

Other topics in the "Obstetrics" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.