Causes of formation
The development of the pathology can begin at different stages of the female body's maturation:
- Prenatal stage: vitamin deficiency and mineral metabolism disorders in the pregnant woman negatively affect bone tissue formation in the female fetus.
- Childhood: poor nutrition, infections and diseases (tuberculosis, rickets), bone injuries, as well as congenital hip dislocation.
- Puberty: endocrine imbalance (decreased estrogen levels against the background of hyperandrogenism) triggers rapid height growth in girls, meaning transverse pelvic parameters fail to keep pace. Narrowing can also be caused by iatrogenic factors — for example, the use of hormonal medications to postpone menstruation in professional athletes.
Classification by form
In Russian medical practice, it is customary to divide anatomically narrow pelves into two large groups depending on their frequency of detection.
Frequently diagnosed forms:
- Transversely contracted;
- Flat (includes simple flat, rachitic flat, and the specific variant with reduction of the direct size of the pelvic cavity's wide part);
- Generally evenly contracted.
Rare forms:
- Asymmetrically contracted (obliquely contracted and obliquely displaced);
- Kyphotic;
- Deformed due to bone tumors, exostoses, or fused fractures;
- Other atypical variants.
Degrees of narrowing
I.F. Zhordania's classification is based exclusively on the length of the true conjugate (conjugata vera):
- Grade I: from 11 to 9 cm.
- Grade II: from 9 to 7 cm.
- Grade III: from 7 to 5 cm (extremely rare in modern obstetric practice).
- Grade IV: 5 cm or less.
Clinical significance: a value of less than 5 cm is interpreted as absolute narrowing. In such a situation, delivering a child through the natural birth canal is physically impossible. For this reason, distinguishing the fourth degree is often considered impractical for real clinical practice.
Epidemiology and mild forms
Modern living conditions have significantly altered the statistics of pelvic anomalies. Thanks to adequate nutrition and a decrease in childhood morbidity, classical flat and generally evenly contracted pelves are detected less and less frequently.
However, due to global acceleration processes, transversely contracted variants and pelves with a shortened direct size of the wide part of the pelvic cavity have come to the forefront. Obstetricians pay special attention to mild forms. In this pathology, one to three dimensions of the lesser pelvis are reduced only slightly (within 10 mm). Despite their apparent harmlessness, such deviations can seriously disrupt the biomechanism of labor and hinder the descent of the fetal presenting part.
Other anomalies: wide pelvis
The opposite anomaly is an anatomically wide pelvis. It is characterized by an increase in all standard parameters by 2 cm or more, which is typical for tall and large women. Approximate external dimensions are: distantia spinarum — 28–29 cm, distantia cristarum — 30–32 cm, distantia trochanterica — 33–34 cm, conjugata externa — 22–23 cm. During a vaginal examination, the doctor records an elongation of the true and diagonal conjugates.
Clinically wide pelvis is a condition where the birth canal is excessively spacious for a specific baby (even if the mother's bones are of normal size, but the fetus is of low weight). The main danger lies in the excessively rapid descent of the fetal head. This can trigger deflexion presentations, precipitate labor, deep tears of the mother's soft tissues, and intracranial hemorrhages in the newborn.
To prevent complications, pain relief is actively used (including epidural anesthesia up to the moment of birth), and the second stage of labor is recommended to be managed with the woman lying on her side.
Briefly: answers to topical questions
What external pelvimetry dimensions are characteristic of a generally evenly contracted pelvis?
A generally evenly contracted pelvis is characterized by a proportional shortening of all external dimensions by 2 cm or more compared to the norm.
- Spinous distance (distantia spinarum) — 24 cm.
- Crest distance (distantia cristarum) — 26 cm.
- Trochanteric distance (distantia trochanterica) — 28 cm.
- External conjugate (conjugata externa) — 18 cm.
A proportional decrease in the longitudinal and transverse dimensions of Michaelis rhomboid is also noted, while maintaining its correct shape.
What external dimensions are characteristic of a simple flat pelvis?
A simple flat pelvis is characterized by a reduction in the external conjugate and the direct size of the pelvic outlet while maintaining normal transverse dimensions.
- Spinous distance (distantia spinarum) — 26 cm.
- Crest distance (distantia cristarum) — 29 cm.
- Trochanteric distance (distantia trochanterica) — 30 cm.
- External conjugate (conjugata externa) — 18 cm.
The shape of Michaelis rhomboid is altered due to a reduction in the vertical (direct) size; with pronounced narrowing, it may take the shape of a triangle.
What are the features of the biomechanism of labor in a generally evenly contracted pelvis?
The biomechanism of labor in a generally evenly contracted pelvis is distinguished by the central position of the head and its descent strictly along the pelvic axis.
- Head molding — when passing through the planes of the lesser pelvis, the head experiences strong compression, elongates sharply in length, and takes on a pronounced dolichocephalic shape.
- Caput succedaneum (caput succedaneum) — a large caput succedaneum forms in the region of the minor fontanelle, which can lead to an error in determining the station of the head.
The course of labor is prolonged, and the appearance of early pushing efforts leads to complications for the mother and fetus.
What objective examination methods are used to diagnose a narrow pelvis?
Clinical and instrumental methods of objective examination are used to diagnose a narrow pelvis.
- External obstetric examination — pelvimetry using a pelvimeter and assessment of the shape and dimensions of Michaelis rhomboid.
- Internal vaginal examination — determination of the shortening of the diagonal conjugate (conjugata diagonalis); in a transversely contracted pelvis, the pubic angle and convergence of the ischial spines are also assessed.
- Instrumental methods — X-ray pelvimetry and magnetic resonance pelvimetry (MRI); CT can be used to identify predictors of obstructed labor and operative delivery.
To memorize the degrees of narrowing according to Zhordania, use the "2 cm step": 11–9–7–5. Anything less than 11 cm is a narrow pelvis. A value of less than 5 cm means absolute narrowing, in which natural childbirth is impossible.
Class and exam questions
Which pelvic dimension is decisive for diagnosing contraction?
The key parameter is the true conjugate (conjugata vera). If it is less than 11 cm, the pelvis is considered anatomically narrow.
What are the risks of latent forms of contraction?
In mild forms, the dimensions are reduced slightly (up to 10 mm), but this may be sufficient to seriously hinder the advancement of the fetal head through the birth canal.
What is the difference between an anatomically and clinically spacious pelvis?
The anatomical pelvis features an objective enlargement of bone dimensions by 2 cm or more. The clinical pelvis is one that is excessively spacious for a specific fetus (even with normal bone dimensions), which threatens rapid labor and trauma.
What else to study on this topic
- Biomechanism of labor in various forms of an anatomically contracted pelvis
- Methods for measuring and calculating the true conjugate
- Clinically contracted pelvis: differences from anatomical contraction
- Indications for cesarean section in bony pelvis anomalies
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