Version June 2024
| 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). |
| Engagement usually occurs in an oblique position (left sacrum anterior, right sacrum anterior, left sacrum posterior, right sacrum posterior). |
| A 45-degree internal (ie, in the birth canal ) rotation occurs. In the anterior positions, a 45-degree backward rotation; in the posterior positions, a 45-degree forward rotation occurs. |
| The anterior buttock descends under the pubic bone and begins to open the vulvar orifice. The posterior buttock pushes the coccyx backwards, distends the perineum, and then becomes exteriorized, which fully releases the anterior buttock. In complete breech presentation, the lower limbs are usually expelled at the same time as the buttocks. |
| The abdomen and the most inferior portion of the fetal chest are expelled. The upper limbs are forced into flexion, shortening the biacromial diameter before assuming an oblique diameter. |
| The biacromial diameter then becomes anteroposterior, the fetal back points to the maternal right or left side, and the head begins to flex. 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. |
| Totally flexed, the head goes through an internal rotation motion to place the occiput under the symphysis. However, if the chin becomes anterior, rotational dystocia will occur. This set of movements is observed as external rotation of the shoulder, with the fetal back facing upwards. |
| The chin, mouth, nose, forehead and finally the occiput emerge. |
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