Bipedalism and human birth: The obstetrical dilemma revisited

Abstract …adaptation to bipedal locomotion decreased the size of the bony birth‐canal at the same time that the exigencies of tool use selected for larger brains. This obstetrical dilemma was solved by delivery of the fetus at a much earlier stage of development. (Washburn 1 ) …there can be no doubt that many of the obstetrical problems of Mrs. H. Sapiens are due to the combination of a narrower pelvis and a bigger head in the species. ( Krogman 2 )