The obstetrical dilemma is a hypothesis to explain why humans often require assistance from other humans during childbirth to avoid complications, whereas most non-human primates give birth unassisted with relatively little difficulty. This occurs due to the tight fit of the fetal head to the maternal birth canal, which is additionally convoluted, meaning the head and therefore body of the infant must rotate during childbirth in order to fit, unlike in other, non-upright walking mammals. Consequently, there is a usually high incidence of cephalopelvic disproportion and obstructed labor in humans. The obstetrical dilemma claims that this difference is due to the biological trade-off imposed by two opposing evolutionary pressures in the development of the human pelvis: smaller birth canals in the mothers, and larger brains, and therefore skulls in the babies. Proponents believe bipedal locomotion (the ability to walk upright) decreased the size of the bony parts of the birth canal. They also believe that as hominids' and humans' skull and brain sizes increased over the millennia, that women needed wider hips to give birth, that these wider hips made women inherently less able to walk or run than men, and that babies had to be born earlier to fit through the birth canal, resulting in the so-called fourth trimester period for newborns (being born when the baby seems less developed than in other animals). Recent evidence has suggested bipedal locomotion is only a part of the strong evolutionary pressure constraining the expansion of the maternal birth canal. In addition to bipedal locomotion, the reduced strength of the pelvic floor due to a wider maternal pelvis also leads to fitness detriments in the mother pressuring the birth canal to remain relatively narrow. This idea was widely accepted when first published in 1960, but has since been criticized by other scientists. History The term, obstetrical dilemma, was coined in 1960, by Sherwood Larned Washburn, a prominent early American physical anthropologist, in order to describe the evolutionary development of the human pelvis and its relation to childbirth and pregnancy in hominids and non-human primates. In the intervening decades, the term has been used broadly among anthropologists, biologists, and other scientists to describe aspects of this hypothesis and related topics. Evolution of human birth Human pelvis The obstetrical dilemma hypothesizes that when hominids began to develop bipedal locomotion, the conflict between these two opposing evolutionary pressures became greatly exacerbated. Because humans are currently