Pathogenesis of disorders in late pregnancy
At the core of all pathological changes in an anatomically narrow pelvis lies a pronounced spatial mismatch. The dimensions of the mother's pelvic ring prove insufficient to allow the fetal presenting part to pass through.
Normally, by the end of the third trimester, the head should descend and engage in the pelvic inlet. However, when contracted, this does not occur. The fetal head cannot overcome the inlet plane and remains completely mobile. In obstetrics, this condition is described as a ballotable head. Such a high position persists right up until the onset of labor.
Because the presenting part is not fixed in the pelvic inlet, the crucial hydrodynamic mechanism is disrupted: the amniotic fluid does not separate into forewaters and hindwaters. Normally, the head presses against the bones to create a kind of barrier, but in a narrow pelvis, this barrier is completely absent.
Clinical presentation in the III trimester
Clinical manifestations in late pregnancy are directly related to the fetus's inability to descend into the pelvic cavity. This leads to a cascade of mechanical displacements of adjacent organs.
First, the fundal height significantly exceeds standard values for the corresponding gestational age. Unable to descend, the uterus enlarges upward, exerting strong pressure on the thoracic organs.
Second, the high standing of the uterine fundus provokes an extremely high elevation of the diaphragm dome. This entails two serious systemic disorders:
- Limitation of lung excursion. Tidal volume decreases because the lungs cannot fully expand during inspiration.
- Cardiac dysfunction. The change in anatomical position and increased intra-abdominal pressure place an additional burden on the pregnant woman's cardiovascular system.
As a result, characteristic symptoms develop: patients complain of pronounced dyspnea. In a narrow pelvis, dyspnea manifests at a significantly earlier stage of pregnancy, is more intense, and persists much longer than in women with normal pelvic dimensions.
Obstetric complications
The key factor in the development of severe obstetric complications is the absence of a tight contact girdle. The contact girdle is the zone of direct contact between the fetal head and the walls of the mother's pelvis. When the head is mobile and positioned high above the pelvic inlet, this protective girdle does not form.
The lack of fixation of the presenting part leads to spatial instability of the fetus within the uterine cavity, provoking the following pathologies:
- Malpresentations and malpositions. The fetus easily changes its axis, leading to transverse and oblique lies.
- Deflexion attitudes. Trying to adapt to the narrow inlet, the head may become deflexed. Pathological engagement variants arise: sincipital, brow, or face presentation.
- Premature rupture of membranes (PROM). Since forewaters and hindwaters are not separated, immense hydraulic pressure is exerted on the amniotic sac, leading to its early rupture.
- Umbilical cord prolapse. This is one of the most formidable complications. When membranes rupture, the fluid rushes outward. Due to the absence of a contact girdle between the head and the pelvic bones, a free space is formed into which an umbilical cord loop easily slips below the fetal presenting part.
Briefly: answers to topical questions
Which pelvimetry dimensions are used to diagnose a transversely contracted pelvis?
Diagnosis of a transversely contracted pelvis via pelvimetry is based on the reduction of external transverse diameters.
- Distantia spinarum (D. spinarum) — 24–25 cm.
- Distantia cristarum (D. cristarum) — 25–26 cm.
- Distantia trochanterica (D. trochanterica) — 28–29 cm.
- Conjugata externa (Conjugata externa) — 20 cm.
- Conjugata diagonalis (Conjugata diagonalis) — 11 cm.
A reduction in the transverse diameter of Michaelis' rhombus (less than 10 cm) and the transverse diameter of the pelvic outlet plane is also detected. The degree of contraction is determined by the magnitude of the transverse diameter of the pelvic inlet plane (diameter transversa).
What is the biomechanism of labor in a transversely contracted pelvis?
The biomechanism of labor in a transversely contracted pelvis depends on the degree of contraction and pelvic dimensions.
- Normal direct diameter of the inlet plane — the head engages in the oblique diameter asynclitically (anterior parietal bone), the sagittal suture shifts posteriorly. Upon descent into the pelvic cavity, the mechanism does not differ from normal (internal rotation, extension, external rotation).
- Reduced transverse and increased direct dimensions — the head enters the inlet with the sagittal suture in the direct diameter. The biomechanism includes flexion, descent to the pelvic floor without internal rotation, extension, and birth.
What degrees of contraction are distinguished in a transversely contracted pelvis?
Degrees of contraction of a transversely contracted pelvis are assessed by the magnitude of the transverse diameter of the pelvic inlet plane (diameter transversa).
| Degree of contraction | Transverse diameter of the inlet (cm) |
|---|---|
| Grade I | 12.5–11.5 (or 12.4–11.5) |
| Grade II | 11,4–10,5 |
| Grade III | less than 10.5 |
These parameters may be determined based on X-ray pelvimetry results.
What is the obstetric management of labor in a transversely contracted pelvis?
Obstetric management of labor in a narrow pelvis excludes "trial labor" in primigravidas and entails expanded indications for cesarean section.
- Conservative management — spontaneous vaginal delivery is permitted only with grade I pelvic contraction, fetal weight up to 3500 g, and occiput presentation.
- Surgical delivery — elective cesarean section is indicated for an anatomically narrow pelvis with grade II–IV contraction.
Pathogenetic logical chain: Narrow pelvis → head does not descend → high uterine fundus → props up the diaphragm → dyspnea. Simultaneously: head ballots → no contact girdle → fluid not separated → upon membrane rupture, cord prolapses.
Class and exam questions
Why is dyspnea more pronounced in pregnant women with a contracted pelvis?
Because the fetal head does not descend into the small pelvis, the fundal height significantly exceeds the norm. This leads to a high standing of the diaphragm, which mechanically limits lung excursion and impairs heart function.
What is the "apposition belt" and why is its absence dangerous?
This is the zone of tight contact between the fetal presenting part and the mother's pelvic bones. If the girdle is absent, amniotic fluid is not separated into forewaters and hindwaters, threatening premature rupture of membranes and umbilical cord prolapse.
What presentations are typical for this pathology?
Due to the impossibility of normal engagement in the pelvic inlet, deflexion head presentations frequently form: sincipital, brow, or face presentations. There is also a high risk of transverse and oblique fetal lies.
What else to study on this topic
- Biomechanism of labor in sincipital, brow, and face presentations
- Management of labor in premature rupture of membranes (PROM)
- Emergency assistance algorithm for umbilical cord prolapse
- Pelvimetry methods and diagnostics of an anatomically narrow pelvis
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