Physical Growth in Newborns

By definition, the newborn period covers the first 28 days of life. This time period is one of amazing changes that the infant will experience as he/she completes the adaptation from inside the uterus to "the great outdoors."

The average newborn dimensions (birth weight, length, and head circumference) are a reflection of parental genetics, maternal health during the pregnancy, and position in the birth order in the family. The average birth weight for a full-term infant is 7 lb. 11 oz. The range of normal weights is from 5 lb. 8 oz. to 8 lb. 13 oz. Infants with birth weights outside of this range warrant a detailed obstetrical history since some issues causing a birth weight outside of this range may persist into infancy and childhood. The weight of male infants is statistically approximately 3 oz. greater than their female counterparts. Many parents discovered that the weight of subsequent children commonly increases with each pregnancy. Finally, larger parents commonly have larger children.

Babies will commonly lose 5%-7% of their birth weight during their first few days of life. This is a reflection of limited intake (breastfed children > bottle-fed children) relative to urine and stool output. However, at 2 weeks of age, their weight should be back to the birth weight. Regardless of mechanism of feeding (breast or bottle) or fluid received (breast milk versus formula), weight loss in excess of 10% represents a significant concern and deserves a thorough and complete evaluation by the newborn's pediatrician. During the first weeks postpartum, newborns will gain approximately 1 ounce per day -- that implies a 1-pound weight gain every two weeks.

A reasonable rule of thumb is that a thriving infant will double his/her birth weight by 4-6 months of age and triple it by their first birthday. Such a rapid rate of weight gain is underscored by the following example. A 7 lb. child at birth will weigh approximately 21 lbs. at 1 year of age. Obviously, this rapid weight gain noted in the first year of life slows down during the second year of life -- a fact that many a parent experiences as their child becomes more rigid and picky in food choices.

The average term infant's length at birth is between 19-21 inches. As would be expected, taller parents tend to produce longer neonates. At the end of their first month, most infants have increased their length by approximately 1 inch. Occasionally, a neonate may "shrink" during the first 2 weeks of age. This is generally due to inaccurate measurement at birth for two reasons:

1. prominent molding of the neonatal head (cone head) will have resolved and
2. "stretching" the neonate during their first set of measurements in the delivery room.

Prior to delivery, the fetus is exposed to maternal levels of the hormone relaxin that increases the "stretch ability" of the maternal pelvis and also allows for some increased laxness for the newly delivered baby.

A full-term neonate will commonly have a head circumference between 13-14 inches. This measurement is approximately 1 inch greater than the same infant's chest and abdominal girth measurements. Biologically, this is a benefit since once the head has been delivered, it is unlikely that the neonate's chest or abdomen will impede a rapid expulsion from the maternal uterus. Some initial head circumference measurements may be affected by the infant's intrauterine position. Prolonged engagement of the infant's head in the maternal pelvis commonly produces a moderate amount of elongation of the back (occiput) of the head commonly termed a cone head configuration. Such a distortion is temporary and generally resolves within the first few days of life. Similarly, an infant in a breech position (either foot or buttock delivered prior to the delivery of the infant's head) will commonly possess a more square-shaped skull. The progressive enlargement of the neonate's skull circumference is a direct reflection of brain growth. An infant whose head circumference measurements reflect either a slower or more rapid rate of growth is a red flag requiring a thorough evaluation.

In order to appreciate the rate of weight, length, or head circumference growth, pediatricians commonly use a growth chart to keep serial records of such measurements. These plotted measurements place the child in statistical percentiles allowing comparison between the neonate and his/her peers. A sudden acceleration or deceleration of growth is obvious utilizing such a system and may be the first indication of serious health issues.

Finally, there is not necessarily a relationship between height/weight and head circumference percentiles. For example, the height and weight may be at the 50% percentile while the head circumference might be in either the 75% or 25% percentile. Documenting consistency in percentile growth is very reassuring in such a situation.

During the first month of life, a newborn makes amazing advances in several areas. One of these areas involves nutrition. A newborn will initially only consume ½ to ¾ oz. per three-hour feeding. By 4 weeks of age, the average child will be consuming 4 oz. approximately every four hours. That is a five-fold increase! Many infants are born with a fine downy hair on the shoulders and upper back. This hair is called lugano and will gradually disappear during the first few months of life. Infants may be born with prominent molding of the head often termed a cone head appearance. This change will resolve over the first week of life. Another developmental change is the appearance of acne of the newborn. This mild case of pimples tends to appear at about 2 weeks of age, stabilize without change for about two weeks, and then resolve over the next two weeks (by 6 weeks of age). Acne of the newborn is not related or predictive of routine acne that develops during adolescence. Acne of the newborn never produces any scarring.

Newborn infants have no sense of day and night; however, by approximately 6 weeks of age they will commonly have started to establish a rhythm in their life. This doesn't imply that they are capable of sleeping through the night without feeding, but it does mean that feeding in the middle of the night can be approached as business and limit any extraneous stimulation. Daytime feedings are an excellent time to have social interaction (such as singing). The distance an infant can see increases from very close to several feet away. (The proposal that infants cannot determine color is not true. They just aren't that interested in colorful objects until about 2- 3 months of age.) Similarly, the human face is not that visually interesting until about 6 weeks of age. Social smiling and cooing are noticed often about 6 weeks of age and should be present by the 2-month-old well child exam.

Premature infants (those born less than 37 weeks' gestation) cover a broad range of early delivery dates. Some infants are born prematurely for no known reason. A spontaneous rupture of the amniotic sac will commonly require attempts to prolong the pregnancy for a period of time. A perinatologist, an ob-gyn specializing in complicated maternal issues during pregnancy, routinely assumes the management of this "holding pattern." Conversely, some maternal or fetal medical issues may require a premature delivery to promote the health and well-being of both individuals. Depending upon the level of prematurity and infant health, the growth parameters may be the same, faster, or slower than a full-term peer. Acquisition of developmental milestones will often be appropriately delayed but may often have fully caught up by 18- 24 months of age. As would be anticipated, the level of prematurity is proportional to the acquisition of developmental milestones. For example, a baby born at 28 weeks' gestational age will take longer to catch up than his peer born at 34 weeks' gestational age. Close monitoring of a premature infant's developmental skills and physical growth are major reasons for routine well-child exams.

Postmature infants (those born after 42 weeks' gestation) commonly have a developmental pattern and rate of physical growth similar to routine-term infants.