Fetal head positioning variants
When evaluating breech presentation, it is critically important to determine the degree of fetal head extension. The further course of labor depends on this spatial relationship. Four main variants of positioning are distinguished:
- Flexed head — the classical state, most favorable for passage through the birth canal.
- Grade I extension — an initial deviation traditionally referred to in obstetric practice as the "military attitude".
- Grade II extension — moderate backward tilting of the head.
- Grade III extension — maximum deviation of the head, figuratively termed the "star-gazing" attitude.
Delivery of the pelvic extremity (1st and 2nd moments)
The biomechanism of labor starts with the advancement of the buttocks through the birth canal.
The first moment consists of the internal rotation of the buttocks. This process begins when the pelvic extremity passes from the wide part of the pelvic cavity into the narrow part. As a result of rotation, the transverse diameter of the buttocks (linea intertrochanterica) is strictly set in the straight diameter of the pelvic outlet. The anterior buttock (facing anteriorly) always descends first, which then positions itself under the pubic arch. It is here that the first fixation point (hypomochlion) is formed — it arises between the lower edge of the maternal pubic symphysis and the fetal anterior iliac bone.
The second moment is the lateral flexion of the fetal lumbar spine. Due to continuous forward movement, the torso flexes, and the posterior buttock smoothly "rolls out" over the perineum. Following it, the anterior buttock is delivered from under the pubic symphysis. Parallel to this process, the fetal shoulders enter the pelvic inlet with their transverse diameter through the same oblique diameter previously traversed by the buttocks, and the back turns anteriorly.
Delivery of the shoulder girdle (3rd and 4th moments)
The third moment is characterized by the internal rotation of the shoulders and external rotation of the trunk. The fetal shoulders align in the straight diameter of the pelvic outlet. Meanwhile, the back turns toward the parturient woman's thigh depending on the initial position: if it is the first (I) position — toward the left thigh, if the second (II) — toward the right. At this stage, the second fixation point is formed: the anterior shoulder (at the border of the upper and middle thirds of the humerus) rests against the pubic arch. The posterior shoulder is located anteriorly to the coccyx, above the perineum.
The fourth moment represents the lateral flexion of the cervicothoracic spine. This movement occurs strictly around the newly formed second fixation point. The result of this stage is the complete delivery of the fetal shoulder girdle and arms.
Delivery of the head (5th and 6th moments)
The final stage requires special attention, as the largest and densest part of the fetus is delivered.
The fifth moment is the internal rotation of the head. Normally, it rotates with the occiput anteriorly. After the shoulder girdle is delivered, the head enters the pelvic inlet. An important spatial detail: it passes through the oblique diameter that is opposite to the diameter in which the shoulders previously advanced. Upon transition from the wide to the narrow part of the pelvis, the head performs its internal rotation.
The sixth moment completes the entire biomechanism. Flexion and final delivery of the fetal head occur.
Briefly: answers to topical questions
What anatomical structures make up the fetal pelvic extremity and what is the mechanism of their rolling out over the perineum?
The fetal pelvic extremity may be represented solely by the buttocks, the buttocks along with the feet, or one or both lower extremities — depending on the variant of breech presentation.
- During internal rotation, the transverse diameter of the buttocks is established in the straight diameter of the pelvic outlet.
- During lateral flexion of the spine, the posterior buttock "rolls out" over the perineum, after which the anterior buttock is delivered from under the pubic symphysis.
- In footling presentation, the lower extremity and buttock facing posteriorly are delivered first, followed by those facing anteriorly.
What obstetric maneuvers are used when delivery of the subsequent fetal head is difficult?
In case of difficulty delivering the subsequent fetal head, manual extraction maneuvers are used, and for additional complications — destructive operations.
- Morice–Levret–Lachapelle maneuver — a manual procedure for extracting the subsequent head, in which tractions are performed along the direction of the pelvic axis.
- Craniotomy — a destructive operation consisting of incising the skull of the subsequent head to reduce its size.
Craniotomy is used in cases of fetal demise and the inability to extract the subsequent head in breech presentation.
To easily remember all six moments of the biomechanism, pay attention to the strict pattern. The process represents a cyclical alternation of two actions: Rotation — Flexion — Rotation — Flexion — Rotation — Flexion. First, the pelvic extremity goes through this cycle, then the shoulder girdle, and finally — the fetal head.
Class and exam questions
What are grades I and III of head extension in breech presentation called in obstetrics?
Grade I extension is called the "military attitude", and grade III (maximum) extension is called the "star-gazing" attitude.
Where does the first fixation point form during the delivery of the pelvic extremity?
The first fixation point (hypomochlion) forms between the lower edge of the maternal pubic symphysis and the fetal anterior iliac bone.
Into which pelvic diameter do the fetal shoulders enter during the second moment of labor?
The shoulders enter the pelvic inlet with their transverse diameter through the same oblique diameter through which the buttocks successfully passed prior to this.
Where does the fetal back turn during the third moment of labor?
The back turns toward the parturient woman's thigh: in position I, it faces the left thigh, and in position II — the right thigh.
What else to study on this topic
- Determination of position and position variety in breech presentation
- Detailed mechanism of internal rotation of the head during transition from the wide to the narrow part of the pelvis
- Obstetric maneuvers during the delivery of arms and the shoulder girdle
- Labor management tactics in excessive head extension ("military attitude" and "star-gazing" attitude)
- Features of the head passing through various planes of the lesser pelvis
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