data is collected and analyzed. Several different fields of study have taken an interest in understanding more about the human birth process and that of human ancestor species. Early brain growth rates Some studies have shown that higher brain growth rates happen earlier on in ontogeny than previously thought, which challenges the idea that the explanation of the obstetrical dilemma is that humans are born with underdeveloped brains. This is because if brain growth rates were largest in early development, that is when the brain size would increase the most. Premature birth would not allow for a much larger head size if most of the growth had already happened. Also, it has been suggested that maternal pelvic dimensions are sensitive to some ecological factors. Maternal heat stress There has been a lot of evidence linking body mass to brain mass, leading to the determination of maternal metabolism as a key factor in the growth of the fetus. Maternal constraints could be largely due to thermal stress or energy availability. A larger brain mass in the neonate corresponds to more energy needed to sustain it. It takes much more energy for the mother if the brain fully develops in the womb. If maternal energy is the limiting factor then an infant can only grow as much as the mother can sustain. Also, because fetal size is positively correlated to maternal energy use, thermal stress is an issue because the larger the fetus, the more the mother can suffer heat stress. Environmental effects Additional studies suggest that other factors may further complicate the obstetrical dilemma hypothesis. One of these is dietary shifts, possibly due to the emergence of agriculture. This can be both due to change in diet as well as the increase in population density since agriculture was developed; more people leads to more disease. Studies have also been done in twins to show that pelvic size may be due more to the environment in which they live than their genetics. Another study disproves the thought that narrower hips are optimized for locomotion because it was found that a Late Stone Age population Southern Africa that survived largely on terrestrial mobility had women who had uncharacteristically small body size with large pelvic canals. Energetics of gestation and growth hypothesis The energetics of gestation and growth (EGG) hypothesis offers a direct challenge to the obstetrical dilemma hypothesis, equating the constraints on gestation