A 28-year-old multigravida at 37 weeks is found to have a malpresentation on routine antenatal examination. On ultrasound, the fetal buttocks are at the lower pole of the uterus with hips flexed and knees extended (frank breech). In contrast, another patient with transverse lie shows the fetal shoulder in the lower uterine segment. Which finding best differentiates the obstetric management approach between these two presentations?