Biomechanism of labor in occiput presentation

External and internal obstetric examination

During labor, continuous monitoring of the mother's general condition (pulse, blood pressure, general well-being, color of mucous membranes) and the fetus is essential. During external uterine palpation, the physician assesses consistency, local tenderness, condition of the round ligaments, and the lower uterine segment.

An important marker is the location of Bandl's ring (contraction ring) above the pubic symphysis. It rises due to the stretching of the lower segment, and its height in centimeters accurately corresponds to the degree of cervical dilation. At full dilation, the ring is located 8–10 cm above the pubic bone.

Vaginal examination is of key importance in assessing labor activity. It is performed at the initial evaluation, upon rupture of amniotic membranes, in case of deviations from the norm, before inducing labor (and every 2 hours thereafter), as well as when indications for emergency delivery arise.

Assessment of the birth canal and amniotic sac

During internal examination, the physician assesses the condition of soft tissues: vaginal capacity, presence of septa, varicosities, scars from previous tears or episiotomies, and the elasticity of the pelvic floor muscles. The cervix (cervix uteri) is evaluated by its shape (preserved, shortened, or effaced), distensibility, thickness and rigidity of the edges, and most importantly, the degree of dilation in centimeters.

The amniotic sac must be examined. Excessive tension during the intervals between contractions indicates polyhydramnios, while a flat amniotic sac (membranes tightly stretched over the fetal head) indicates oligohydramnios, which can delay the progress of labor. Normally, the discharged amniotic fluid is clear or slightly turbid due to lanugo, desquamated epithelium, and vernix caseosa. The presence of meconium indicates pathology.

Determination of the presenting part and moments of labor

In cephalic presentation, the obstetrician palpates anatomical landmarks: sutures and fontanelles. The position, variety, type of engagement (synclitic or asynclitic), and the current moment of the labor mechanism (e.g., flexion or extension of the head) are identified specifically by the location of the sagittal suture, anterior, and posterior fontanelles.

Dynamics of fetal head descent through the pelvic planes

Assessing the position of the fetal head relative to the classic planes of the minor pelvis is the basis of labor management. There are six consecutive stages of descent:

1. Above the pelvic inlet: the head is high, freely ballotable, or merely pressed against the inlet. The linea terminalis, sacral promontory, sacrum, and pubic bone are accessible on internal examination.
2. Small segment in the inlet: the lowest pole has crossed the plane, the head is fixed, but the largest segment remains above the inlet. During the fourth external Leopold maneuver, the obstetrician's fingertips converge, while the palms diverge. Internally, the sacral hollow and pubic symphysis are palpated.
3. Large segment inumia (in the inlet): the largest diameter is firmly fixed and has passed the plane of the inlet. Externally, the fingertips diverge, and the palms are parallel. Internally, the lower 2/3 of the pubic bone, the sacral hollow, and the ischial spines are accessible.
4. In the wide part of the pelvic cavity: the head has completely cleared the inlet. Externally, it is no longer palpable above the pelvis. Internally, the lower third of the pubic bone, the 4th–5th sacral vertebrae, and the ischial spines are palpated.
5. In the narrow part of the pelvic cavity: descent continues, the ischial spines are identified with difficulty, and crowning begins.
6. At the pelvic outlet: internal rotation is complete, the head has descended to the pelvic floor and is ready for crowning and birth.

Evaluation of uterine contractility

Fetal advancement is driven by labor activity. It is assessed by tone, intensity, duration, and frequency of contractions using tocography and cardiotocography (CTG).

In the first stage of labor, normal tone (the lowest intrauterine pressure between contractions) does not exceed 10–12 mm Hg, and intensity (maximum pressure) increases from 25 to 50 mm Hg. In the active phase, the normal frequency is about 4 contractions per 10 minutes, and total uterine activity is 200–240 Montevideo units. For objective monitoring and detection of abnormalities, a partograph is maintained.

Class and exam questions

What else to study on this topic

• Indications for emergency delivery on the part of the mother and fetus.
• Rules for managing and interpreting the partograph.
• CTG assessment: accelerations and fetal response to contractions.
• Diagnosis of intrauterine infection and hypoxia based on the characteristics of the amniotic fluid.
• Formulating a complete clinical diagnosis in labor.

Other topics in the "Obstetrics" section

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