Anatomical structure of the bony pelvis
From an obstetric perspective, the pelvis is considered the main bony structure forming the birth canal. In addition, it performs a crucial supporting function and serves as a link between the trunk and the lower extremities.
The pelvis of an adult female is a sturdy bony ring formed by four elements:
- Two hip bones, or innominate bones (ossa coxae).
- The sacrum (os sacrum).
- The coccyx (os coccygis).
It is the internal space of this bony structure—the lesser pelvis—that serves as the channel through which the fetus passes during birth.
Pelvic inlet plane and obstetric diameters
To assess the capacity of the birth canal, physicians rely on the bony structures (hip bones, sacrum, coccyx) and specific dimensions of the pelvic inlet plane.
Main obstetric diameters:
- Anteroposterior diameter (true conjugate, conjugata vera)—the sagittal diameter connecting the sacral promontory (promontorium) and the most prominent point on the inner surface of the pubic symphysis. Normally, it is 11–11.5 cm.
- Transverse diameter—the distance between the furthest points of the arcuate (terminal) lines of the right and left iliac bones.
- Oblique diameters (right and left)—intersecting lines running from the sacroiliac joint on one side to the iliopubic eminence on the opposite side. Their length is 12–12.5 cm.
Additionally, the anatomical conjugate (conjugata recta), measuring 9–11.5 cm, and the anteroposterior diameter of the pelvic outlet, measuring 11 cm, are distinguished.
Axes, angles, and gender differences
The progression of the fetus in labor strictly follows the anatomical curves of the birth canal. On the sagittal section of the pelvis between the pubic symphysis (symphysis pubica) and the sacrum, the pelvic axis (axis pelvis) is defined. This is an arch-shaped line, open anteriorly. It passes through the centers of all anteroposterior diameters of the lesser pelvic planes and indicates the exact direction of fetal movement.
An important parameter is the inclination of the pelvis (inclinatio pelvis). It is formed at the intersection of the horizontal plane with the pelvic inlet plane (the line passing through the true conjugate).
The pelvis also exhibits distinct sexual dimorphism. The pubic arch (arcus pubicus), formed by the inferior rami of the pubic bones, is 90–100° in women. By comparison, the subpubic angle (angulus subpubicus) in men is significantly sharper, at approximately 70°.
Anatomical types of the lesser pelvis
Depending on the shape of the pelvic inlet and bone structure features, four main types of pelvis are distinguished in obstetrics:
- Gynecoid (gynecoid)—the normal female type. The inlet shape is round or slightly transverse-oval, which is optimal for childbirth.
- Android (android)—the male-type pelvis. The inlet shape is wedge-like, resembling a heart shape on playing cards, and characteristically narrows toward the pubic symphysis.
- Anthropoid (anthropoid)—the inlet shape is longitudinal-oval. In this case, the anteroposterior diameter of the inlet predominates over the transverse one.
- Platypelloid (platypelloid)—the flat pelvis. The inlet shape is markedly transverse-oval, with a shortened anteroposterior inlet diameter and an increased transverse diameter.
The fetus as the object of labor: types of presentation
In addition to pelvic anatomy, fetal position is critical for successful delivery. Depending on which part of the fetus faces the pelvic inlet, various types of presentation (praesentatio) are distinguished.
Cephalic presentation is classified by the degree of flexion or extension of the head:
- Occiput presentation—a flexion type, typical and physiological.
- Sinciput, brow, and face presentations—variants in which the head is in varying degrees of extension.
Breech presentation includes the following variants:
- Frank breech (incomplete).
- Complete breech.
- Footling breech.
Class and exam questions
What is the true conjugate and how is it measured?
The true conjugate (conjugata vera) is the anteroposterior diameter of the pelvic inlet plane. It represents the shortest distance between the sacral promontory (promontorium) and the innermost prominent point of the pubic symphysis. Normally, this diameter is 11–11.5 cm.
Why is it necessary to know the pelvic axis?
The pelvic axis (axis pelvis) is an imaginary arch-shaped line passing through the centers of all anteroposterior diameters of the pelvic planes. It indicates the physiological pathway along which the fetus progresses through the birth canal during labor.
How does the gynecoid pelvic type differ from the platypelloid type?
The gynecoid pelvis is the classical female type with a round or slightly transverse-oval inlet shape. The platypelloid (flat) pelvis has a markedly transverse-oval inlet, in which the anteroposterior diameter is shortened while the transverse diameter is conversely increased.
What variants of fetal breech presentation exist?
Obstetrics distinguishes three main variants of breech presentation: frank breech (incomplete), complete breech, and footling breech presentation.
What else to study on this topic
- Structural features of the fetal skull: sutures and fontanelles.
- Dimensions of the fetal head and the concept of head segments.
- Basic obstetric terminology: fetal lie (situs) and position (positio).
- Fetal attitude (visus) and its significance in the biomechanism of labor.
- Planes of the lesser pelvis and their detailed dimensions (beyond the inlet plane).
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