Obstetrics and Gynecology - A.M. Gromova 2000

Anomalies of the Bony Pelvis
Features of the Biomechanism of Labor in a Simple Flat Pelvis

The course, prognosis, and Management of Labor in cases of a contracted pelvis depend on both the pelvic shape and the degree of contraction. During labor, it is essential to consider the fetal HEAD dimensions, the character of uterine contractions, as well as The Mechanism of engagement and moldability of the fetal head.

An Anatomically Contracted Pelvis is rarely an indication for elective Cesarean Section. It is warranted only in cases of grade III–IV pelvic contraction, exostoses or Bone tumors in the pelvic cavity, significant pelvic deformities obstructing fetal passage, previous rupture of the Pubic Symphysis or other pelvic injuries, presence of urogenital or rectovaginal fistulas, etc.

In grade I and II pelvic contractions, an elective Cesarean section is performed when a contracted pelvis is combined with Post-term Pregnancy, fetal macrosomia, breech presentation, genital malformations, fetal malposition, Infertility, history of stillbirth, or other obstetric and extragenital pathologies.

In grade I pelvic contraction, labor is typically managed vaginally in the initial stages.

Considering the specific course of labor in women with a contracted pelvis, the parturient should remain on strict bed rest During the first stage of labor to prevent early rupture of membranes.

The parturient should be positioned on her side to facilitate the engagement of the fetal head into the pelvic inlet via the small oblique diameter.

The physician must closely monitor the General condition of the parturient, her pulse, Blood pressure, bladder and bowel function, the pattern of uterine contractions, and the state of the birth canal. To maintain her physical strength, high-calorie and vitamin-rich foods are prescribed: sour cream, hot meat broth, chocolate, sweet strong tea or coffee, fruit juices, lemon, and others. Psychotherapy is also administered.

If uterine inertia develops, medical labor augmentation is prescribed after previously verifying that the fetal head dimensions correspond to the maternal pelvic size. If the amniotic membrane rupture interval exceeds 12 hours, antibacterial agents are administered to the parturient for the Prevention of intrapartum chorioamnionitis.

The management of labor includes functional Assessment of the pelvis, continuous electronic fetal monitoring if available, partogram plotting, prevention of intrauterine Fetal Hypoxia and intrapartum Hemorrhage, and liberal use of Episiotomy. Labor should be managed in an active-expectant manner. Upon the appearance of signs of grade II–III fetopelvic disproportion or other complications (such as acute hypoxia), delivery by Cesarean section is indicated.

When managing labor with a generally uniformly contracted pelvis, potential complications during the delivery process must be anticipated. If for 3–4 hours the fetal head remains mobile or pressed against the pelvic inlet without any forward progression—despite adequate uterine contractions and full cervical dilation—and Changes in the fetal Heart rate occur, a Cesarean section is indicated to save the lives of both the mother and the fetus.

Complications may also arise during the Second Stage of labor: overdistention of the lower uterine segment, entrapment of soft Tissues, signs of fetal hypoxia, progressive edema of the external genitalia, Hematuria, and others. In such cases, active intervention is required, The Nature of which depends on the specific case and obstetric conditions. When choosing the method of intervention, one must be guided by the fundamental principle of obstetrics—to preserve the life and health of both the mother and the fetus.

In flat rachitic pelvis, to facilitate the passage of the fetal head through the narrowed pelvic inlet, Walcher’s hanging position can be used. It increases the conjugate diameter of the pelvic inlet by almost 0.5 cm.

It should always be remembered that excessive conservatism in managing labor with a contracted pelvis, especially with an unfavorable obstetric history, can lead to severe complications (endometritis, Uterine rupture, vesicovaginal fistulas, intrauterine fetal demise) and necessitate destructive obstetric operations. It is crucial, while maintaining a sensible and attentive conservative-expectant approach, not to miss the opportune moment for a Cesarean section.

Two concepts of a contracted pelvis are distinguished: the anatomically contracted pelvis and the clinically contracted pelvis. A functionally (clinically) contracted pelvis is defined as one that presents difficulties or obstacles to the normal course of labor.

A clinically contracted pelvis occurs in 0.5–1.7% of all deliveries, and its frequency remains comparable to previous years, which can be attributed to an increase in fetal dimensions.

All cases of disproportion between the fetal head and the maternal pelvis, regardless of the absolute pelvic dimensions, should be classified as clinically contracted pelvis.

The signs of a clinically contracted pelvis include:

- abnormal head engagement;

- insufficient head moldability;

- positive Vasten’s (Thangemeister’s) sign;

- symptoms of Urinary Bladder compression;

- absence of forward progression of the fetal head, especially with full cervical dilation and strong uterine contractions;

- symptoms of impending uterine rupture.

Two clinical signs are extremely important for diagnosing maternal-fetal disproportion: inadequate progress of cervical dilation and slow descent of the fetal head. In cases of fetopelvic disproportion, the cervix at full dilation does not retract over the fetal head; instead, it often becomes edematous and passively hangs into the Vagina, creating a false impression of incomplete dilation.

The earliest and most reliable sign of a clinically contracted pelvis is the specific mechanism of head engagement, which should be regarded as an adaptive mechanism allowing the fetal head to overcome the resistance presented by the maternal pelvis.

If the fetal head has descended into the pelvis, the issue of cephalopelvic disproportion is ruled out. If the head has not yet entered the pelvic cavity, attention is paid to Vasten's sign or the degree to which the head overhangs the pubic symphysis. This sign becomes reliable after the rupture of membranes, once the head is firmly engaged at the pelvic inlet.

Using the fingertips, the obstetrician moves upward from the pubic bone toward the fetal head. If the head and pelvis are proportionate, the head lies below the level of the symphysis (Vasten's sign is negative), indicating that vaginal delivery is feasible. If the head is level with the symphysis (edge-to-edge), it points to a relative disproportion that may be overcome with adequate fetal Skull molding and active labor. In cases of cephalopelvic disproportion, where the fetal head projects above the symphysis (Vasten's sign is positive), the prognosis for labor is doubtful.

At the same time, the presence or absence of Vasten's sign depends on the peculiarities of head engagement. With posterior asynclitism, Vasten's sign will invariably be positive. Conversely, with anterior asynclitism, Vasten's sign will be absent even in the presence of a significant mismatch between the maternal pelvis and the fetal head. From the above, it is clear that a positive or edge-to-edge Vasten's sign should be regarded as a manifestation of a clinically narrow pelvis. However, the absence of Vasten's sign does not guarantee favorable proportions between the maternal pelvis and the fetal head.

The Classification of a clinically narrow pelvis based on the degree of disproportion between the maternal pelvis and the fetal head (R.I. Kalganova, 1965) identifies three degrees of disproportion, characterized by the following features:

Grade I disproportion (relative disproportion):

1) Specific features of head engagement and the mechanism of labor typical of the given form of pelvic contraction;

2) adequate fetal skull molding.

These factors, combined with vigorous labor activity, help the fetal head overcome pelvic resistance and ensure a favorable labor outcome. The duration of labor is somewhat longer than usual.

Grade II disproportion (marked disproportion between the maternal pelvis and the fetal head):

1) specific features of head engagement and the mechanism of labor typical of the given form of pelvic contraction;

2) pronounced, severe skull molding;

3) prolonged arrest of the head in a single pelvic plane;

4) symptoms of bladder compression (dysuria, hematuria);

5) presence of the flush Bandl's ring (or relative/flush Vasten sign).

Prolonged arrest of the fetal head in a single pelvic plane and extended labor duration pose significant risks to the mother's health and contribute to high perinatal mortality. Expectant management of labor in the presence of Grade II fetopelvic disproportion is not always advisable. The combination of Grade II disproportion with an anatomically contracted pelvis, advanced maternal age, an unfavorable obstetric history (stillbirth, prolonged infertility), and other labor complications should serve as an indication for abdominal delivery.

Grade III fetopelvic disproportion (marked or absolute disproportion):

1) features of head engagement characteristic of a given type of pelvic contraction, although a mechanism of engagement atypical for this form of anatomically narrow pelvis frequently occurs;

2) pronounced molding of the fetal head or absence of its molding capacity, especially in a post-term fetus;

3) positive Vasten sign;

4) severe symptoms of bladder compression;

5) premature onset of involuntary, ineffective expulsive efforts;

6) absence of progressive descent of the fetal head with full cervical dilation and vigorous uterine activity;

7) symptoms of impending uterine rupture.

In Grade III fetopelvic disproportion between the mother's pelvis and the fetal head, vaginal delivery of a live fetus is impossible. The only appropriate method of delivery in such cases is a Cesarean section.

To identify fetopelvic disproportion and determine the degree of size discrepancy, a functional assessment of the pelvis during labor must be performed. The functional assessment should be preceded by its anatomical evaluation and estimation of fetal size using conventional Methods. In the absence of obvious signs of disproportion, the core management principle for a clinically narrow pelvis is a cautious, time-limited expectant approach. The signs of a clinically narrow pelvis should also be regarded as functional diagnostic tests.

To ensure proper and timely diagnosis of a clinically narrow pelvis, it is essential to closely monitor the laboring woman's condition, the correspondence between her pelvic dimensions and the fetal head, head descent, uterine contraction patterns, uterine tone, and bladder function.

Vaginal examination not only allows assessment of cervical dilation and head descent, but also helps identify the fine details of the mechanism of engagement, the characteristics of cranial molding, and the condition of the soft tissues in the birth canal. Once the cervix is significantly or fully dilated and the amniotic membranes have ruptured, the Hofmeier-Mueller sign is evaluated. For this purpose, the obstetrician inserts a hand into the vagina and asks the woman to bear down 2-3 times. If no movement of the fetal head is felt during bearing down, fetopelvic disproportion should be suspected. These findings, combined with external examination results and, wherever possible, radiological data regarding the true dimensions and shape of the pelvic contraction, help determine the presence and degree of disproportion, thereby guiding the choice of the most rational management of labor.

The character of uterine activity is a crucial factor contributing to the timely detection of disproportion and determining the outcome of labor. Even with minor degrees of disproportion, adequate molding and engagement characteristics must be supported by vigorous and effective uterine contractions.

At the same time, a clinically narrow pelvis is frequently associated with labor abnormalities: primary or secondary uterine inertia, hypertonic labor, or premature pushing efforts while the fetal head is still free-floating. Among the causes of primary uterine inertia, infantilism and degenerative changes in the myometrium of multiparous women with a complicated obstetric history can play a significant role. Secondary uterine inertia typically arises from the exhaustion of uterine contractility due to preceding vigorous contractions. Conversely, hypertonic labor and premature, unproductive pushing efforts occur due to excessive irritation of the receptors in the cervix and lower uterine segment caused by prolonged arrest of the head in a single pelvic plane.

In cases of weak labor activity, medical stimulants should be used while carefully considering potential contraindications. Until the degree of disproportion is established, the administration of oxytocin as a selective uterine stimulant is strictly contraindicated.

In the presence of hypertonic labor and the absence of signs of grade III disproportion, antispasmodics and analgesics should be used to regulate contractions. Simultaneously, prevention of fetal hypoxia is recommended, as the duration of labor in this pathology is shorter than usual. Furthermore, the fetus is adversely affected by the disproportion itself, which causes excessive compression of the head.

Since functional pelvic assessment relies on an expectant approach, the question naturally arises as to the acceptable duration of this expectancy. This is a difficult question, and answering it correctly requires an individualized approach in each specific case. However, it is certain that excessively prolonged expectancy is usually the result of an obstetric error leading to the delayed detection of existing pathology. Clinical experience shows that in the majority of women with fetopelvic disproportion, the degree of disproportion and the choice of delivery method can be clarified during The First stage of labor. For instance, if with grade I disproportion (where clinical manifestations are limited to engagement features characteristic of the specific type of contracted pelvis and adequate cranial molding) the signs of disproportion do not progress as labor advances, the continuation of expectant management is unquestionable.

The progression of signs of disproportion between the maternal pelvis and the fetal head during labor indicates grade II or III disproportion, which in many cases warrants cesarean delivery during the first stage of labor, without awaiting a functional pelvic assessment In the second stage.

In cases of a clinically narrow pelvis caused by unfavorable head engagement (high straight arrest, persistent occipitoposterior face presentation, brow presentation), expectant management can generally be limited to the first stage of labor. Thus, if vigorous uterine activity and adequate cervical dilation fail to yield a tendency toward favorable Rotation of the head or further flexion in brow presentation, continued expectancy is unjustified, and delivery should be accomplished abdominally without performing a pelvic assessment in the second stage of labor.

If, for whatever reason, the diagnosis of a clinically narrow pelvis is made belatedly, functional pelvic assessment in the second stage of labor must be limited. Specifically, in the absence of head progression despite full cervical dilation and vigorous uterine activity for 1 hour in multiparous women and 1.5 hours in primiparas, grade III disproportion is unquestionable, and labor should be terminated by cesarean section. Additionally, the duration of expectancy is determined by the condition of both the mother and the fetus.

When managing labor via the natural birth canal in women with a narrow pelvis, perineal incision (mediolateral episiotomy is preferable) is indicated to reduce maternal and fetal trauma, along with the prevention of postpartum hemorrhage.

Procedures such as vacuum extraction and Obstetric Forceps are not widely used in cases of a narrow pelvis due to their traumatic nature for both the mother and the fetus.

During the second stage of labor in a narrow pelvis, delivery may be complicated by shoulder dystocia (difficult delivery or impaction of the shoulders), where the anterior shoulder becomes trapped behind the symphysis pubis after the head is born. As a result, further descent of the fetus through the birth canal halts. At this point, the posterior shoulder is either tightly wedged in the sacral hollow or located above the sacral promontory.

Although the fetal head has been delivered and the fetus attempts to breathe, its chest is severely compressed by the walls of the birth canal, preventing the Lungs from expanding. Placental Circulation is impaired due to changes in uterine volume and, occasionally, cord compression. The fetus can rapidly succumb to asphyxia.

According to various authors, the incidence of shoulder dystocia ranges from 0.5 to 0.8% of all deliveries.

Shoulder dystocia may be caused by difficulties in rotating the shoulders from the transverse to the direct diameter of the pelvic outlet. One of The most significant risk factors contributing to shoulder dystocia is a fetal weight of 4000 g or more.

Several methods have been proposed to correct shoulder dystocia. When this complication occurs, adequate anesthesia and a sufficient mediolateral episiotomy are indicated.

The fetal head is grasped with both hands over the right and left temporomalar regions and gently pulled downward until the anterior shoulder rests under the pubic arch at the junction of its upper and middle thirds. Then, the head is lifted upward and anteriorly to release the posterior shoulder. Subsequent fetal traction is performed along the pelvic axis.

To deliver the shoulders, moderate downward pressure is applied to the uterine fundus and the suprapubic area, which facilitates the engagement of the anterior shoulder beneath the pubic bone. Suprapubic pressure should be directed backward and laterally toward one side or the other of the mother's abdomen. This maneuvers the biacromial diameter into one of the oblique diameters of the pelvis, thereby facilitating the passage of the shoulder girdle. To deliver the anterior shoulder, two fingers are sometimes inserted into the fetal axilla.

If the posterior shoulder has not yet descended into the sacral hollow, the fetal head must also be grasped and traction applied upward (anteriorly) and as laterally to the midline as possible, advancing the anterior shoulder forward and allowing the posterior one to descend. After this, the maneuver described above is performed—namely, delivery of the anterior shoulder followed by the posterior one.

It is possible to use the R. Kirch maneuver, in which, after manually displacing the shoulders upward via a vaginal hand, the anterior or posterior shoulder is rotated by 45° using finger pressure, guiding the most accessible shoulder along the posterior surface. Rotational maneuvers may also incorporate moderate pressure in the fundal area (Kristeller maneuver), though not as an independent intervention, but as an adjunctive measure.

Rotational maneuvers described by S. Woods are widely used. Their principle is that by rotating the posterior shoulder anteriorly, it is delivered, while the anterior shoulder shifts into the sacral hollow and its subsequent extraction presents no special difficulty. There are several modifications of this method. Two or more fingers inserted into the vagina from the fetal chest side press against the anterior surface of the posterior shoulder, causing a 180° rotation of the fetus toward its back.

This maneuver can also be performed by inserting fingers from the fetal back side and rotating toward its chest. In this case, fingers may be inserted posteriorly into the axilla of the posterior shoulder while executing a 180° rotation. Simultaneously, traction should be applied along the pelvic axis. Rotation must be performed with extreme caution due to the risk of Brachial Plexus injury.

In cases of difficulty delivering the shoulder girdle, grasping, flexing, and delivering the posterior arm is successfully employed due to the generous capacity of the sacral hollow. If the anterior shoulder is still not released, it is rotated 180° into the posterior position and extracted.

Among the methods that allow for reducing the biacromial diameter of the shoulder girdle is fracture of the anterior or posterior clavicle. As a corrective method for shoulder dystocia, this technique does not have A large number of proponents, but it is applied to a living fetus if other methods fail. Pressure on the clavicle in this case should be applied from the side of the fetal Sternum to prevent pleural injury.

The described manipulations can be supplemented with methods that expand the maternal pelvis. Special attention should be paid to maximum flexion of the laboring woman's hip joints. This method is the least traumatic and at the same time highly effective. It results in the straightening of the sacrum relative to the lumbar spine, upward Displacement of the symphysis pubis by approximately 8 cm, and a decrease in the pelvic inclination angle.

CPD and the methods used for its correction pose a high risk to both mother and fetus. Cervical tears, deep lacerations of the vagina and Perineum, and massive hemorrhage can further complicate the situation.

Clavicular and humeral fractures, as well as upper extremity paresis and paralysis, are often detected in newborns. Excessively forceful head rotation during the delivery of the shoulders—and especially traction on the head when the shoulder girdle is fixed—is extremely dangerous. This damages the vertebral Arteries, frequently leading to respiratory disorders in newborns. The Cells of the spinal respiratory center, which play a crucial role in regulating the newborn's breathing, are located at the CIV level in the fetus. Spinal Cord injury at this level can lead to neurogenic asphyxia. Furthermore, the vertebral arteries, which supply blood to the Medulla Oblongata and the cervical spine, pass through the transverse processes of the cervical vertebrae. Even minor damage to the vertebral artery wall can trigger a spasm, disrupt vertebrobasilar circulation, and lead to sudden fetal death during labor or The Development of paralysis in the newborn.

Prevention of a contracted pelvis should begin during the prenatal period. A proper, well-balanced diet for the pregnant woman is of utmost importance for fetal development and The formation of the nervous, endocrine, and skeletal systems.

Physical Exercise in early childhood, a nutritious vitamin-rich diet, an appropriate daily routine for teenage girls, and compliance with school hygiene rules are the primary preventive measures that help reduce the incidence of various pelvic anomalies.

It is difficult to overestimate Structure/19.html">The Importance of preventing Rickets in early childhood, as the most severe forms of contracted pelvis can be caused by this disease.

Preventing tuberculosis is also of great significance in averting pelvic anomalies, as its dissemination can lead to Bone and joint lesions of various localizations (spondylitis, coxitis, gonitis).

Not only tuberculosis, but also other infectious osteomyelitides can cause lesions of the pelvic joints followed by their deformation (ankylotic, coxalgic pelves). Preventing poliomyelitis is crucial, because unilateral lower limb paralysis—a potential consequence of this pathology—leads to the formation of an obliquely contracted pelvis.

Various severe systemic infections suffered during childhood can contribute to the underdevelopment of the entire body, the Reproductive System, and the pelvis.

Every pregnant woman with a pronounced form of contracted pelvis must be referred in a timely manner for specialized examination and appropriate obstetric care.

Review Questions

1. Prognosis of Labor course depending on the degree of pelvic contraction.

2. How is Vasten's sign determined?

3. Degrees of cephalopelvic disproportion according to R.I. Kolganova.

a) signs of the 1st degree of disproportion;

б) signs of the 2nd degree of disproportion;

в) signs of the 3rd degree of disproportion.

4. Management of labor depending on the degree of disproportion between the mother's pelvis and the fetal head.

5. Prevention of contracted pelvis.

6. Which changes in the bony pelvis are classified as anomalies?

7. Classification of Contracted pelves by the type of contraction.

8. Classification of contracted pelves by the degree of contraction.

9. Causes of anatomically contracted pelvis.

10. Diagnosis of anatomically contracted pelvis.

11. How do pointed and pendulous abdomen develop during pregnancy?

12. Course of pregnancy in contracted pelvis.

13. Clinical Course of Labor in contracted pelvis.

14. Features of the biomechanism of labor in a generally evenly contracted pelvis.

15. Features of the biomechanism of labor in a rachitic flat pelvis.

16. Features of the biomechanism of labor in a simple flat pelvis.