During the first year of life, infants experience rapid physical growth. Most babies triple their birth weight and increase in length by as much as 50 percent by their first birthday. A newborn’s brain is only about 25 percent of its approximate adult size. The brain will continue to grow and develop throughout childhood and reach its peak sometime during adolescence – around age 15.

Most healthy newborns are born weighing anywhere from 6 pounds, 2 ounces (2,812 grams) to 9 pounds, 2 ounces (4,173 grams). On average, a newborn’s length typically ranges from 19 to 21 inches (48.3 to 53.3 centimeters). Many factors can affect a newborn’s size at birth including length of pregnancy, multiple births and/or birth order, gender and health in addition to the mother’s health and nutrition during pregnancy.

During the first week of life, a newborn’s weight may decrease as much as 10 percent below initial birth weight. Generally, this is common and should not be cause for concern. This weight loss is usually related to the normal fluid loss after birth and the infant’s ability to feed. This ability improves gradually and by the second week of life, most babies regain or exceed their initial birth weight.

A newborn’s skull is flexible and consists of several bones that will gradually fuse together over time. It includes two soft spots called fontanels where cranial bones have not yet fused. This flexibility allows the head to squeeze through the narrow birth canal and can give infants born vaginally a cone shaped head. The flexibility of the fontanels also accommodates a baby’s rapidly growing brain during the first 12 to 18 months of life.

This flexibility early in life can make an infant susceptible to developing flat spots on the head (positional plagiocephaly). This condition is not serious and may be treated by changing a baby’s head position regularly or with the use of special headbands or molded helmets. A newborn’s head may also have lumps or bruises as a result of the trauma of delivery. This is normal and often lessens over a few days or weeks. 

In addition to the head, other features such as the face, ears, nose or jaw, may be distorted by the position they were in before birth. As the trauma of delivery subsides, an infant’s features typically change significantly. The white part of a newborn’s eyes may appear bloodshot initially. This condition, called subconjunctival hemorrhage, is also caused by the trauma of delivery and typically disappears without medical treatment after several days.

The genitals (sex organs) of both male and female newborns may be swollen at birth. This often subsides on its own as a baby grows. However, in some cases it may be caused by an inguinal hernia – a condition in which part of the intestines juts out through a weak spot in the groin (where the abdomen meets the thigh). An inguinal hernia usually requires surgical treatment.

In most cases, a newborn’s eye color will change overtime. Most babies are born with bluish-gray eyes that gradually lighten or darken as pigmentation of the iris (color part of the eye) takes on a permanent color around age 3 to 6 months. However, babies born with brown eyes do not experience a change of eye color.

A newborn’s skin also undergoes change. Most healthy babies typically have deep red or purple skin and bluish hands and feet at birth. The skin usually lightens a little in the next couple of days and may become initially dry or flaky. Maternal hormones can cause mild skin rashes that resemble acne on babies, which usually clear on their own in a few weeks.

Newborn skin is sensitive and it is not usually recommended to give newborns a tub bath until the umbilical cord stump has fallen off. This typically occurs by the time a baby is 8 weeks old. A sponge bath or damp washcloth – using mild baby soap applied to the water if needed – is the best way to clean a newborn until the cord stump falls off. Also, baths in a tub or sink should be deferred until the circumcision site (in boys) is completely healed. A bath two to three times a week during a baby’s first year of life is all that is needed to maintain hygiene. More frequent bathing is discouraged since it may dry an infant’s delicate skin.

Parents and other caregivers also should keep the infant’s umbilical cord stump as clean and dry as possible. If the area becomes wet or soggy during a sponge bath, it should be gently dried. Parents are urged to consult with their baby’s doctor if the navel area becomes reddened or starts to bleed or if a foul odor or discharge develops as these can be signs of infection.    

Jaundice in healthy newborns is common and often appears around the second or third day after birth and may disappear without treatment within one to two weeks. This is caused by excess bilirubin (a waste product) that accumulates in the body because a baby’s undeveloped liver is not able to remove it effectively at first.

Pediatric health experts generally recommend that infants gain at least 2 pounds (907 grams) by their third month to support healthy growth and development. During the first three months, babies can grow in length by as much as 1.4 inches (3.5 cm) per month and their head size can increase in circumference by 0.79 inch (2 cm) per month. Throughout the baby’s first year, the pediatrician will monitor growth in weight, height and head size for any abnormalities.

Sometime between the fifth and eighth months, an infant’s first tooth usually appears. The two bottom front teeth are normally the first to appear. By the end of the first year, babies typically have six to eight teeth. Signs indicating an infant is teething include excessive drooling, swollen gums, irritability, trouble sleeping and a low-grade fever. Dental care is important even before an infant starts teething. After feedings, parents are urged to gently run a damp washcloth over a baby’s gums to keep bacteria from building up. When teeth begin to appear, a small, soft-bristled toothbrush can be used. 

During the first six months of infancy, proper nutrition can be limited to breast milk or formula. After that, a combination of solid foods – starting with iron-fortified baby cereals at first – and either breast milk or formula should be given until a baby is 1 year old.