and parturition to the energy restrictions of the mother. It has been shown in studies using professional athletes and pregnant women, that there is an upper limitation to the amount of energy a woman can produce before it causes deleterious effects: approximately 2.1x their basal metabolic rate. During pregnancy the growing brain mass and length in the neonate correspond to more energy needed to sustain it. This results in a competing balance between the fetus's demand for energy and the maternal ability to meet that demand. At approximately nine months gestation, the fetus's energy needs surpasses the mother's energy limitation, correlating with the average time of birth. The newly born infant can then be sustained on breast milk, which is a more efficient, less energy demanding mechanism of nutrient transfer between mother and child. Additionally, this hypothesis demonstrates that, contrary to the obstetrical dilemma, an increased pelvic size would not be deleterious to bipedalism. Studying the running mechanics of males and females, it was shown that an increased pelvic size related to neither an increased metabolic nor structural demand on a woman. Obstetrical dilemma revisited The obstetrical dilemma hypothesis has also been challenged conceptually based on new studies. The authors argue that the obstetrical dilemma hypothesis assumes that human, and therefore hominid, childbirth has been a painful and dangerous experience through the species' evolution. This assumption may be fundamentally false as many early analyses focused on maternal death data from primarily females of European-descent in Western Europe and the United States during the 19th and 20th century, a limited population. In a recent study a covariation between human pelvis shape, stature, and head size is reported. It is said that females with a large head possess a birth canal that can better accommodate large-headed neonates. Mothers with large heads usually give birth to neonates with large heads. Therefore, the detected pattern of covariation contributes to ease childbirth and has likely evolved in response to strong correlational selection. A recent study aimed to evaluate the original ideas under the 'obstetrical dilemma' and provide a detailed, more complex explanation for the tight fetopelvic fit observed in humans. They propose the original obstetrical dilemma hypothesis remains valuable as a foundation to explain the complex combination of evolutionary, ecological, and biocultural pressures that constrain maternal pelvic form and fetal size. References Obstetrics Human evolution Pelvis Biological hypotheses