Peculiarities of labor course
Labor in a contracted pelvis is often accompanied by untimely rupture of amniotic fluids, as the fetal head does not press tightly against the pelvic inlet. This creates conditions for ascending infection, the development of chorioamnionitis, and placentitis. Due to the lack of adequate pressure on the lower uterine segment, labor anomalies such as uterine inertia or uncoordinated contractions frequently occur.
Dangerous complications
In case of inadequate management of labor, prolonged compression of soft tissues by the fetal head leads to their ischemia and necrosis. After 5–7 days, this may result in the formation of urogenital or rectovaginal fistulas. There is also a threat of uterine rupture due to overdistension of the lower segment and the risk of fetal traumatic injuries, including intracranial hemorrhage.
Mechanisms of labor
The biomechanism of labor is determined by the type of pelvic contraction:
- Transversely contracted pelvis: with a normal anteroposterior diameter, the fetal head engages asynclitically. If the transverse diameters are reduced, the head may engage in the anteroposterior diameter, which, in case of cephalopelvic disproportion, leads to persistent occiput posterior or deep transverse arrest.
- Generally evenly contracted pelvis: characterized by maximal flexion of the head at the inlet, which passes through the planes of the lesser pelvis in the oblique diameters.
Briefly: answers to topical questions
Which pelvic shapes belong to rare forms of anatomically contracted pelvis?
Rarely encountered forms of anatomically contracted pelvis include several types of deformities.
- Obliquely displaced and obliquely contracted pelvis
- Pelvis with exostoses, post-fracture pelvis, and bone tumors
- Kyphotic pelvis
- Assimilated pelvis
- Funnel-shaped pelvis
- Osteomalacic pelvis
- Spondylolisthetic pelvis
- Split (anteriorly open) pelvis
- Pelvis constricted by ovarian, uterine, or pelvic tumors
These forms are significantly less common than the frequent ones (transversely constricted, flat, and generally evenly contracted pelvis) and collectively account for about 4.4% of contracted pelvis cases.
What anatomical changes are characteristic of a kyphotic pelvis?
A kyphotic pelvis develops in the presence of kyphosis of the lower spine, when the sacrum is displaced posteriorly, rotating around the transverse axis.
- Upper parts of the iliac bones — diverge
- Lower parts and ischial tuberosities — converge
- Anteroposterior diameter of the inlet plane — increases
- Dimensions of the outlet (especially transverse) — sharply decrease
As a result, the pelvis acquires a funnel shape (dimensions decreasing from top to bottom), and obstacles to the passage of the fetal head arise in the narrow part of the pelvic cavity and the outlet plane. Cervical and thoracic kyphosis, balanced by lumbar lordosis, does not affect pelvic dimensions.
What are the causes and diagnostic signs of an obliquely contracted (obliquely displaced) pelvis?
An obliquely displaced (asymmetric) and obliquely contracted pelvis results from pathologies that disrupt musculoskeletal symmetry during childhood.
- Scoliosis
- Coxitis
- Unilateral hip dislocation
- Shortening of one of the lower extremities
- Poorly healed fractures of the pelvis and leg
- Functional disorders of one leg
One of the diagnostic signs is the evaluation of Michaelis' rhombus: in an obliquely contracted pelvis, marked asymmetry of the rhombus is noted — one half is smaller than the other, and the diagonals are unequal.
What morphological characteristics does an osteomalacic pelvis have?
An osteomalacic pelvis is associated with osteomalacia—softening of the bones due to impaired mineralization, which leads to characteristic deformities.
- Protrusio acetabuli (lat.) — deformation of the acetabular rim by the femoral head, disappearance of the teardrop figure contour
- Triradiate pelvic deformity — particularly pronounced in childhood vitamin D deficiency
- Looser zones (Looser zones) — radiolucent, unmineralized linear areas with sclerotic margins, located, among other places, in the region of the pubic symphysis
- Bone becomes soft and bends
These changes form a constricted and deformed pelvis.
What are the anatomical features of a funnel-shaped pelvis?
A funnel-shaped pelvis is mentioned in sources as one of the rare forms of contracted pelvis, although its specific anatomical features are not separately described. The funnel shape (decreasing dimensions from top to bottom) is cited in sources in connection with the kyphotic pelvis, rather than as an independent description of a funnel pelvis.
How does the shape of Michaelis' rhombus change in an obliquely contracted pelvis?
In an obliquely contracted pelvis, the shape of Michaelis' rhombus becomes markedly asymmetric.
- One half of the rhombus — is smaller than the other
- Diagonals of the rhombus — are unequal to each other
This distinguishes the obliquely contracted pelvis from the generally evenly contracted pelvis, in which the shape of the rhombus remains regular but proportionally reduced, and from the rachitic flat pelvis, where the upper half of the rhombus is flattened with an obtuse upper angle.
What is the delivery management tactic in severe forms of pelvic deformities (spondylolisthetic, osteomalacic)?
In severe pelvic deformities that create a mechanical obstacle to the birth of the fetus, Caesarean section is indicated. According to current clinical guidelines, an obstacle to the birth of a child on the part of the birth canal is considered to be an anatomically contracted pelvis of degree II constriction or higher, as well as pelvic bone deformity. Modern management tactics for labor in a contracted pelvis involve abandoning "trial labor" in primiparas and significantly expanding the indications for Caesarean section, especially with concomitant obstetric pathology; spontaneous delivery is allowed only if pelvic constriction does not create mechanical obstacles.
Remember the "two-hour" rule: fetal head arrest in a single plane for longer than this time is a direct pathway to soft tissue necrosis and fistula formation.
Class and exam questions
Why do hemorrhages in the postpartum period often occur with a contracted pelvis?
Prolonged and protracted labor depletes the uterine energy reserve, leading to uterine hypotonia and impaired placental separation.
What is the danger of using the Kristeller maneuver in a contracted pelvis?
The use of auxiliary force in a contracted pelvis repeatedly increases the risk of pubic symphysis rupture and severe fetal trauma.
What is asynclitic engagement?
This is a mechanism where the sagittal suture is displaced posteriorly or anteriorly, allowing the fetal head to overcome the pelvic inlet by means of inclination.
What else to study on this topic
- Differential diagnosis of contracted pelvis forms.
- Criteria for choosing the delivery method (Cesarean section vs. vaginal delivery).
- Particularities of the postpartum management in the presence of fistulae.
- Management tactics for persistent occipito-posterior position or high direct arrest.
- Prevention of fetal infection in prolonged rupture of membranes.
Master the entire topic in the "Obstetrics and Gynecology" course
This is a summary. The course covers everything tested in classes and exams.
- Full textbook on "Rare forms of contracted pelvis" and 973 other pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 21 video lessons on the subject
Other topics in the "Obstetrics" section
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.