Gestation length in humans is believed to be shorter than most other primates of comparable size. The gestation length for humans is 266 days, or eight days short of nine months, which is counted from the first day of the woman's last menstrual period. During gestation, mothers must support the metabolic cost of tissue growth, both of the fetus and the mother, as well as the ever-increasing metabolic rate of the growing fetus. Comparative data from across mammals and primates suggest that there is a metabolic constraint on how large and energetically expensive a fetus can grow before it must leave the mother's body. It is thought that this shorter gestation period is an adaptation to ensure the survival of mother and child because it leads to altriciality. Neonatal brain and body size have increased in the hominin lineage, and human maternal investment is greater than expected for a primate of our body mass. The obstetrical dilemma hypothesis suggests that in order to successfully undergo childbirth, the infant must be born earlier and earlier, thereby making the child increasingly developmentally premature. The concept of the infant being born underdeveloped is called altriciality. Humans are born with an underdeveloped brain; only 25% of their brains fully developed at birth as opposed to non-human primates where the infant is born with 45–50% brain development. Scientists have believed that the shorter gestation period can be attributed to the narrower pelvis, as the baby must be born before its head reaches a volume that cannot be accommodated by the obstetric canal. The obstetrical dilemma posits that this is due to the biological trade-off imposed by two opposing evolutionary pressures in the development of the human pelvis. As human ancestor species (hominids) developed bipedal locomotion (the ability to walk upright), decreasing the size of the bony birth canal, they also developed ever larger skulls, which required a wider obstetrical pelvic area to accommodate this trend in hominid infants.