The chin, mouth, nose, forehead and finally the occiput emerge. This set of movements is observed as external rotation of the shoulder, with the fetal back facing upwards. However, if the chin becomes anterior, rotational dystocia will occur. Totally flexed, the head goes through an internal rotation motion to place the occiput under the symphysis. The anterior shoulder is wedged under the symphysis pubis at the level of the acromion, while the posterior shoulder pushes the coccyx backwards and is expelled, followed by expulsion of the anterior shoulder. The biacromial diameter then becomes anteroposterior, the fetal back points to the maternal right or left side, and the head begins to flex. The mechanism of labor consists of the cardinal movements of engagement, descent, flexion, internal rotation, and the accessory movements of extension and external rotation. The upper limbs are forced into flexion, shortening the biacromial diameter before assuming an oblique diameter. The abdomen and the most inferior portion of the fetal chest are expelled. In complete breech presentation, the lower limbs are usually expelled at the same time as the buttocks. The posterior buttock pushes the coccyx backwards, distends the perineum, and then becomes exteriorized, which fully releases the anterior buttock. The anterior buttock descends under the pubic bone and begins to open the vulvar orifice. In the anterior positions, a 45-degree backward rotation in the posterior positions, a 45-degree forward rotation occurs. The bitrochanteric diameter descends obliquely with slight posterior asynclitisim ( the posterior buttock descends ahead of the anterior buttock, the intergluteal cleft is closer to the pubis than to the sacrum).Įngagement usually occurs in an oblique position (left sacrum anterior, right sacrum anterior, left sacrum posterior, right sacrum posterior).Ī 45-degree internal (ie, in the birth canal ) rotation occurs.
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