Quick review: the narrow part of the lesser pelvis cavity
Before analyzing the outlet plane, it is advisable to review the narrow part of the lesser pelvis cavity. This plane is bounded posteriorly by the sacrococcygeal joint, anteriorly by the inferior margin of the pubic symphysis, and laterally by the ischiadic spines (spinae ischiadicae).
- The anteroposterior diameter is about 11.5 cm (from the sacrococcygeal joint to the inferior margin of the symphysis).
- The transverse diameter is about 10.5 cm (between the ischiadic spines); this is the narrowest transverse diameter of the lesser pelvis cavity, comparable in significance to the conjugate vera.
Regular review of the material is necessary because anatomical dimensions are easily forgotten and require systematization.
The pelvic outlet plane
The pelvic outlet plane (apertura pelvis inferior) is the lower boundary of the lesser pelvis cavity through which the passage of the fetus is completed in labor. Its boundaries are:
- posteriorly — the tip of the coccyx;
- anteriorly — the pubic arch (arcus pubicus), formed by the inferior rami of the pubic bones;
- laterally — the inner surfaces of the ischiadic tuberosities (tubera ischiadica) and the sacrotuberous ligaments.
Unlike the planes located above it, the outlet plane has a shape close to a rhombus rather than an oval or a circle.
Dimensions of the pelvic outlet plane
The dimensions of the pelvic outlet plane have an important obstetric feature — they change during labor due to the mobility of the coccyx.
- The anteroposterior diameter is between the tip of the coccyx and the inferior margin of the pubic arch. Outside of labor, it is about 9.5 cm, but during labor, due to the deflection of the coccyx posteriorly, it increases by approximately 2 cm and reaches 11.5 cm.
- The transverse diameter is between the inner surfaces of the ischiadic tuberosities and is about 11 cm.
The ability of the coccyx to deflect posteriorly under the action of the fetal presenting part is an important biomechanical factor of the birth act.
The pelvic axis
The pelvic axis (axis pelvis) is the line connecting the midpoints of the anteroposterior diameters of all classical planes of the lesser pelvis: the inlet, the broad part, the narrow part, and the outlet. Geometrically, it is a curve passing through the points that bisect the anteroposterior diameters.
Features of the pelvic axis:
- in the upper parts of the pelvis, it has a direction close to vertical;
- in the lower parts, it curves anteriorly, following the shape of the sacral curvature;
- it has the form of a smooth arc concave anteriorly.
Clinical significance: the pelvic axis determines the trajectory of movement of the fetal presenting part during labor and is used in assessing the biomechanism of labor.
The angle of pelvic inclination
The angle of pelvic inclination (inclinatio pelvis) is the angle formed by the pelvic inlet plane and the horizontal plane in the upright body position.
- Normally in a woman, this angle is approximately 45–55°.
- The angle is acute.
- Its value depends on body position, posture, constitution, and age.
An excessively large or small angle of pelvic inclination can affect the biomechanism of labor by altering the relationship between the pelvic axis and the axis of the birth canal and the inlet plane.
Key points
- The pelvic outlet is bounded by the tip of the coccyx, the pubic arch, and the ischiadic tuberosities.
- The anteroposterior diameter of the outlet outside of labor is 9.5 cm; during labor, it increases to 11.5 cm due to the deflection of the coccyx.
- The transverse diameter of the outlet is about 11 cm between the ischiadic tuberosities.
- The transverse diameter of the narrow part (≈10.5 cm) is the smallest transverse diameter of the pelvic cavity.
- The pelvic axis connects the midpoints of the anteroposterior diameters of all planes and has an arc shape.
- The pelvic axis determines the trajectory of fetal advancement in labor.
- The angle of pelvic inclination is formed by the inlet plane and the horizontal, normally 45–55°.
- Knowledge of the pelvic planes, axis, and angle of inclination is necessary to understand the biomechanism of labor.