The internationally agreed on definition for humans is an infant during the first 2 weeks after birth, although this does not hold for some countries (e.g., US) where it can extend to the first month after birth. Compared to non-human primates, both human brain and body weights at birth are higher although the duration of human pregnancy (37-40 weeks) is only slightly longer than that of the great apes. In fact, humans have a larger mean birthweight (3300 grams or slightly more) than would be expected (2200 gram) for a primate of the same size. Such a difference, combined with a relatively larger head, means that delivery of the human fetus at term is fraught with dangers for both the mother and her offspring. While human pregnancy is absolutely longer than for the common chimpanzee (Pan troglodytes), it is shorter relative to life span, and especially to periods of physical immaturity, than for any other primate. Thus, human pregnancy appears to have be shortened in relative during evolution, perhaps to ensure a better ‘fit’ between fetal size and the dimensions of the female pelvis. A consequence, however, a human newborn is in a fetal-like state with regard to non-vital functions, and thus one that requires continuous contact with the caregiver and feeding on demand.
See Active sleep, Altricial, Anterior fontanelle, Apgar score, Asymmetrical tonic neck posture or response (ATNR), Babinski response, Basal ganglia (development), Brain (or encephalon), Clonus, Corticobulbar tract (CBT), Delivery position (or presentation), Embryo, Environment of evolutionary adaptedness, Epiglottis, Fetus, Infancy/infant, Moro response, Neonatal death, Newborn imitation, Optical tomography, Perinatal, Perinatal focal damage, Precocial, r- and k-selection theory