At birth, the newborn, or neonate, enters the neonatal period, which is the first 6 weeks or so of a newborn’s life, when it is considered a neonate and undergoes drastic changes. Infancy is the human life stage beginning at the end of the neonatal period and ending at around two years. Rapid development continues throughout infancy.
Immediately after birth, neonates go through a rapid transition, with increased heart and respiratory rates and a lower body temperature. This temperature change is because of the difference between the uterine environment temperature and the environmental temperature after birth, which is much cooler than human body temperature. The neonate stabilizes into a regular cycle of sleeping and then waking every three to four hours to eat. Respiration is initially difficult, and the neonate breathes rapidly to fill its lungs, around 45 breaths per minute. At around two weeks, respiratory rate stabilizes to approximately 12 breaths per minute.
A neonate goes through several circulatory adaptations after the abrupt clamping and cutting of the umbilical cord by the physician or midwife. The following changes occur: the umbilical vessels collapse, the proximal umbilical arteries form the superior vesical artery, which supplies blood to the bladder, and other vessels become ligaments. In a neonate's heart, blood flow causes two flaps of tissue to close an opening between left and right atria—the foramen ovale. In most infants, these flaps fuse permanently by the end of the first year.
Additional changes occur to the immune system, circulatory system, and endocrine system. Neonates have weak immune responses but carry strong maternal immunity gained through the placenta. For approximately six months, infants are well protected from most infectious diseases, including measles, diphtheria, and polio. Neonates are less able than adults to thermoregulate. Brown fat stored during fetal development protects them from hypothermia. Gradually, infant metabolic rates increase, and infants store more fat. Kidneys in neonates are incompletely developed. Infants thus have high rates of water loss and must take in more fluid relative to body weight than adults.
Infants undergo developmental milestones on a relatively predictable timeline, with variations attributable to genetic, socioeconomic, and cultural differences. At six weeks, an infant can typically smile and hold up its head. At three months, the infant can begin thumb-sucking (some infants start thumb-sucking in utero), and at four months, the infant can grasp objects. Vision improves, and infants follow bright objects with their eyes. A five-month-old can usually sit supported. Between six months and one year, infants sit without support, crawl, and pull themselves up; many begin walking. A one-year-old understands speech and may say several words. Physical development slows after one year, but fine motor skills improve. Between one and two years of age, brain activity changes rapidly as infants master communication skills and constantly learn about the world around them.