size and changes in pelvic structure are the major contributors of obstructed labor in modern humans. It is also common for obstructed labor in humans to be caused by the fetus's broad shoulders. However, morphological shifts in pelvic structure still account for the inability of a fetus to pass effectively through the birth canal without major complications

Other primates have a wider and straighter birth canal that allows a fetus to pass through more effectively. Mismatch between birth canal size and