Determining if There is Enough Room.
The calf is in anterior longitudinal presentation, dorso-sacral position, with normal posture. One leg is pulled by one person until the pastern is 15 cm outside the vulva. This means that the point of the shoulder has passed the iliac shaft. It is held there. If the second leg can be pulled equally far (again by one person) outside the vulva, both shoulders will have passed the iliac shafts of the pelvis and the narrowest portion of the birth canal. Since the pectoral girdle is the greatest diameter of the calf, the entire calf can now be delivered per vaginam by traction, while rotating the calf to accommodate the hips of the calf.
Drost M (1974)