Female pelvic shapes
In clinical practice, completely "pure" types of bony ring structure are rare; physicians most often deal with mixed variants. Classically, four basic shapes are distinguished:
- Gynecoid (found in half of women). The pelvic inlet has a rounded or transversely oval outline. The walls are smooth, the pubic arch is obtuse, and the transverse diameter of the narrow cavity is at least 10 cm. The sacrospinous notch is distinct and rounded.
- Android (about 30% of cases). Resembles the male pelvic structure. The inlet is heart-shaped, and the cavity itself narrows downward like a funnel. The bones are thickened, the pubic angle is acute, and the ischial spines prominently project. The sacral hollow is virtually flat.
- Anthropoid (approximately 20%). Characterized by a longitudinally oval inlet, where the anteroposterior diameter significantly predominates over the transverse. The cavity is elongated and narrow, the pubic angle is acute, and the sacrosciatic notch is large.
- Platypelloid (less than 3%). A shallow, flat pelvis, flattened from top to bottom. The inlet is transversely oval (anteroposterior diameters are reduced), the sacrum is markedly deflected posteriorly, and the pubic arch is obtuse.
The fetus as an object of labor: skull and sutures
To monitor the infant's descent through the birth canal, the obstetrician relies on sutures and fontanelles. The infant's skull is formed by two frontal, two parietal, two temporal, as well as the occipital, sphenoid, and ethmoid bones.
Key landmarks during a vaginal examination:
- Sagittal (longitudinal) suture — runs between the right and left parietal bones. Anteriorly, it abuts the anterior fontanelle; posteriorly, the posterior fontanelle.
- Frontal suture — separates the frontal bones (in newborns, they have not yet fused).
- Coronal suture — the junction of the frontal and parietal bones. It runs perpendicularly to the sagittal and frontal sutures.
- Occipital (lambdoid) suture — connects the occipital bone with the parietals.
At the intersection of these lines lie the anterior and posterior fontanelles — the primary identification landmarks in labor.
Fetal dimensions and head segments
To assess the proportionality between the fetus and the maternal pelvis, it is important to know the dimensions of a term infant.
Head parameters:
- Vertical dimension (from the vertex to the hyoid bone) — 9.5 cm (corresponding circumference is 32 cm).
- Largest transverse diameter (between the parietal eminences) — 9.5 cm.
- Smallest transverse diameter (between the furthest points of the coronal suture) — 8 cm.
Trunk parameters:
- Biacromial diameter — 12 cm (circumference 35 cm).
- Bitrochanteric diameter — 9–9.5 cm (circumference 27–28 cm).
The head is conventionally divided into segments. The largest segment is the maximum circumference with which the head passes through the pelvic planes. Its size varies depending on the presentation: in occipital presentation, it is the circumference of the suboccipitobregmatic diameter; in sincipital presentation, the anteroposterior diameter; in brow presentation, the large oblique diameter; and in face presentation, the vertical diameter. Any part of the head smaller than the largest segment is called the small segment.
Fetal topography in the uterus
The correct position of the infant in the uterine cavity is the key to a normal course of labor. Physicians evaluate two main parameters:
- Attitude (habitus) — the relationship of the extremities and head to the trunk. Normally, the infant is folded into an ovoid: the back is flexed, the chin is pressed to the chest, the legs are drawn up to the abdomen, and the arms are crossed. The broader part (pelvis) is located in the uterine fundus, while the narrower part (occiput) points toward the outlet. Disturbances of this posture (e.g., neck extension) occur in 1–2% of cases and complicate the process.
- Lie (situs) is the relationship between the longitudinal axis of the fetus and the long axis of the uterus. It can be longitudinal (situs longitudinalis), when the axes coincide; transverse (situs transversus), when they intersect at a right angle; and oblique (situs obliquus), when the angle of intersection is acute.
It is easy to remember the sutures by their names and locations: the sagittal runs along the head like an arrow, the coronal wraps around it transversely like a crown (diadem), and the frontal and occipital speak for themselves — located anteriorly and posteriorly.
Class and exam questions
What pelvic shape is considered optimal for childbirth?
The gynecoid shape. It is observed in approximately 50% of women and is characterized by a rounded or transversely oval inlet, smooth walls, and an obtuse pubic arch, which is most favorable for fetal passage.
What is the largest segment of the fetal head?
This is the largest circumference with which the head passes through the planes of the lesser pelvis. Its diameter directly depends on the degree of flexion or extension of the fetal neck (type of presentation).
How does fetal lie differ from fetal attitude?
Lie (situs) is the relationship of the fetal axis to the uterine axis (longitudinal, transverse, oblique). Attitude (habitus) is the posture of the infant itself, i.e., the relationship of its arms, legs, and head to its own trunk.
What else to study on this topic
- Biomechanics of head passage in various types of presentation (occipital, brow, face).
- Structural features and exact dimensions of the fetal skull fontanelles.
- Clinical significance and management of labor in mixed female pelvic shapes.
- Causes and obstetric consequences of abnormal fetal attitude.
- Diagnostic methods for fetal lie, position, and presentation during pregnancy and labor.
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