The neonatal period extends from birth to somewhere between 2 weeks and 1 month.

Immediately after the baby is born, uterine contractions force blood, fluid, and the placenta from the mother's body. The umbilical cord—the baby's lifeline to it's mother—is now severed. Without the placenta to remove waste, carbon dioxide builds up in the baby's blood. This fact, along with the actions of medical personnel, stimulates the control center in the brain, which in turn responds by triggering inhalation. Thus the newborn takes its first breath. As the newborn's lungs begin to function, the bypass vessels of fetal circulation begin to close. The bypass connecting the atria of the heart, known as the foramen ovale, normally closes slowly during the first year.

During this period the body goes through drastic physiological changes. The most critical need is for the body to get enough oxygen as well as an adequate supply of blood. (The respiratory and heart rate of a newborn is much faster than that of an adult.)

Umbilical cord: The umbilical cord of a newborn is bluish-white in color. After birth, the umbilical cord is normally cut, leaving a 1–2 inch stub. The umbilical stub will dry out, shrivel, darken, and spontaneously fall off within about 3 weeks. Occasionally, hospitals may apply triple dye to the umbilical stub to prevent infection, which may temporarily color the stub and surrounding skin purple.

Colour and Skin: A newborn's skin is oftentimes grayish to dusky blue in color. As soon as the newborn begins to breathe, usually within a minute or two of birth, the skin's color returns to normal tones. Newborns are wet, covered in streaks of blood, and coated with a white substance known as vernix caseosa, which is believed to act as an antibacterial barrier. The newborn may also have Mongolian spots, various other birthmarks, or peeling skin, particularly at the wrists, hands, ankles, and feet.

Weight and Height: A newborn's shoulders and hips are narrow, the abdomen protrudes slightly, and the arms and legs are relatively short. The average weight of a full-term newborn is approximately 7 ½ pounds (3.2kg), but can be anywhere from 5.5–10 pounds (2.7–4.6kg). The average total body length is 14–20 inches (35.6–50.8cm), although premature newborns may be much smaller. The Apgar score is a measure of a newborn's transition from the womb during the first ten minutes of life.

Head: A newborn's head is very large in proportion to the rest of the body, and the cranium is enormous relative to his or her face. While the adult human skull is about 1/8 of the total body length, the newborn's is twice that. At birth, many regions of the newborn's skull have not yet been converted to bone. These "soft spots" are known as fontanels; the two largest are the diamond-shaped anterior fontanel, located at the top front portion of the head, and the smaller triangular-shaped posterior fontanel, which lies at the back of the head.

During labor and birth, the infant's skull changes shape to fit through the birth canal, sometimes causing the child to be born with a misshapen or elongated head. This will usually return to normal on its own within a few days or weeks. Special exercises sometimes advised by physicians may assist the process.

Body hair: Some newborns have a fine, downy body hair called lanugo. It may be particularly noticeable on the back, shoulders, forehead, ears and face of premature infants. Lanugo disappears within a few weeks. Likewise, not all infants are born with lush heads of hair. Some may be nearly bald while others may have very fine, almost invisible hair. Some babies are even born with a full head of hair. Amongst fair-skinned parents, this fine hair may be blond, even if the parents are not. The scalp may also be temporarily bruised or swollen, especially in hairless newborns, and the area around the eyes may be puffy.

Genitals: A newborn's genitals are enlarged and reddened, with male infants having an unusually large scrotum. The breasts may also be enlarged, even in male infants. This is caused by naturally-occurring maternal hormones and is a temporary condition. Females (and even males) may actually discharge milk from their nipples, and/or a bloody or milky-like substance from the vagina. In either case, this is considered normal and will disappear in time.