Human childbirth has often been described as uniquely difficult and dangerous because the size of the fetal head is nearly the same size as the maternal bony pelvis. This tight fit between the fetal head and maternal pelvis has long been thought to be the result of two competing forces acting on the female pelvis. Efficient bipedalism is thought to require a narrow pelvis, but increasing brain size in the human lineage demands a larger birth canal to accommodate a big-brained human neonate. The evolution of the obstetrical dilemma and the human birth process has been inferred from fossil remains. However, several key assumptions of this important anthropological model are now being questioned by new research.