Characteristics of passing through the pelvic cavity
After overcoming the pelvic inlet, the fetal head flexes. Further descent often proceeds according to the classic scenario, including internal rotation and extension. However, alternative variants are also possible: labor in the occiput posterior presentation or in the sincipital presentation, if the deflexed state of the head persists.
Due to the increased anteroposterior diameters of the pelvic cavity and outlet after a prolonged delay at the inlet, the phenomenon of "stormy" labor may develop. The head passes through the birth canal extremely quickly, increasing the risk of traumatic injuries. At the same time, its configuration approachesbrachycephalic, and a large birth tumor—caput succedaneum—forms on the parietal bone.
Biomechanism of labor in a simple flat pelvis
At the plane of the pelvic inlet, head engagement occurs similarly to that in a rachitic flat pelvis. Within the pelvic cavity itself, events may unfold according to two scenarios:
- Delivery via the occipital presentation type.
- Occurrence of persistent transverse arrest of the sagittal suture, when internal rotation is hindered, and the head descends to the pelvic outlet with the sagittal suture positioned transversely.
If rotation still does not occur, dangerous complications arise: secondary uterine inertia and acute fetal hypoxia.
Indications for elective Caesarean section
Modern obstetric tactics involve abandoning the practice of "trial labor" in primiparae with an expansion of indications for surgery. The list of strict indications for Caesarean section includes:
- Anatomically contracted pelvis of grade II–IV or grade I with a macrosomic fetus (from 3500 g).
- Exostoses, bone tumors of the lesser pelvis, and severe post-traumatic deformations.
- Previous uterine scar, rupture of the pubic symphysis, or history of urogenital fistulas.
- Unfavorable obstetric history (infertility, IVF, absence of living children).
- Post-term pregnancy, breech presentation, deflexion head presentations, and chronic hypoxia.
Monitoring of labor activity and the parturient's condition
Vaginal delivery is permissible only with grade I constriction, occipital presentation, and low fetal mass (up to 3500 g), provided there are no mechanical obstacles. Mandatory monitoring covers:
- General parameters (blood pressure, pulse, respiration, temperature).
- Characteristics of contractions using a partograph and sensors.
- Fetal heart rate, head descent, and the degree of its molding.
A critically important sign is monitoring urination. The inability to urinate and bladder distension signal fetopelvic disproportion due to compression of the urethra or overstretching of the lower uterine segment.
Briefly: answers to topical questions
What is the biomechanism of labor in a simple flat pelvis?
The biomechanism of labor in a simple flat pelvis depends on which plane the fetal head is located in.
- At the pelvic inlet — the mechanism of head engagement is similar to labor in a rachitic flat pelvis.
- In the pelvic cavity — delivery may occur via occipital presentation or the formation of persistent transverse arrest of the sagittal suture.
Transverse arrest occurs when internal rotation is difficult and the head descends to the plane of the pelvic outlet with the sagittal suture in the transverse diameter. If rotation still does not occur, secondary uterine inertia and acute fetal hypoxia develop.
What are the features of the biomechanism of labor in a generally equally contracted pelvis?
The features of the biomechanism of labor in a generally equally contracted pelvis lie in 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, it experiences severe compression, elongates sharply, and acquires a pronounced dolichocephalic shape.
- Caput succedaneum (caput succedaneum) — forms in the area of the small fontanelle and reaches large sizes.
The course of such labor is prolonged. A large caput succedaneum can descend to the plane of the pelvic outlet, creating the risk of error in determining the station of the head during a vaginal examination.
First point for vaginal delivery: grade I pelvis, weight up to 3.5 kg, occipital presentation, and no disproportion.
Class and exam questions
In what cases are spontaneous deliveries permitted with a contracted pelvis?
Vaginal delivery is possible only with grade I pelvic contraction, fetal mass up to 3500 g, occipital presentation, and if the contraction does not create mechanical obstacles to fetal advancement.
What does urinary retention in a parturient indicate?
Bladder distension and inability to empty it independently indicate a discrepancy between the dimensions of the fetal head and the mother's pelvis due to mechanical compression or overstretching of the lower uterine segment.
When is oxytocin labor augmentation permitted?
Oxytocin augmentation is permitted strictly in the absence of disproportion between the fetal head and the woman's pelvis, necessarily against the background of adequate pain relief.
What else to study on this topic
- Features of head engagement in a rachitic flat pelvis
- Diagnosis of clinically contracted pelvis during labor
- Methodology and diagnostic value of vaginal examination
- Use of the partograph to assess the character of labor activity
- Causes and prevention of acute fetal hypoxia
Master the entire topic in the "Obstetrics and Gynecology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete guidelines on "Biomechanism of labor in a contracted pelvis" and 973 more 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.