Proportional Changes

During the first year of life, especially the initial 6 months, growth is very rapid. Infants gain 150 to 210 g (≈5–7 oz) weekly until approximately age 5 to 6 months, when the birth weight has at least doubled. An average weight for a 6-month-old child is 7.3 kg (16 pounds). Weight gain slows during the second 6 months. By 1 year of age, the infant’s birth weight has tripled, for an average weight of 9.75 kg (21.5 pounds). Infants who are breastfed beyond 4 to 6 months of age typically gain less weight than those who are bottle fed, yet their head circumference is more than adequate. There is evidence that breastfed infants tend to self-regulate energy intake. This self-regulation of intake with breastfeeding (vs. formula [bottle] feeding) is believed to have further significance in the development of childhood obesity and subsequent cardiovascular disease (Grummer-Strawn, Mei, and Centers for Disease Control and Prevention [CDC], 2004; Schack-Nielsen and Michaelsen, 2006). Researchers also found that infants who were bottle fed in early infancy were more likely to empty the bottle or cup of milk in late infancy than infants who were breast fed (Li, Fein, and Grummer-Strawn, 2010).

Height increases by 2.5 cm (1 inch) a month during the first 6 months of life and also slows during the second 6 months. Increases in length occur in sudden spurts, rather than in a slow, gradual pattern. Average height is 65 cm (25.5 inches) at 6 months and 74 cm (29 inches) at 12 months. By 1 year of age, the birth length has increased by almost 50%. This increase occurs mainly in the trunk rather than in the legs and contributes to the characteristic physique of the infant.

Head growth is also rapid. During the first 6 months, head circumference increases approximately 1.5 cm (0.6 inch) per month, but the rate of growth declines to only 0.5 cm (0.2 inch) monthly during the second 6 months. The average size is 43 cm (17 inches) at 6 months and 46 cm (18 inches) at 12 months. By 1 year, head size has increased by almost 33%. Closure of the cranial sutures occurs, with the posterior fontanel fusing by 6 to 8 weeks of age and the anterior fontanel closing by 12 to 18 months of age (average, 14 months).

Expanding head size reflects the growth and differentiation of the nervous system. By the end of the first year, the brain has increased in weight about 2.5 times. Maturation of the brain is exhibited in the dramatic developmental achievements of infancy (Table 10-1). Primitive reflexes are replaced by voluntary, purposeful movement, and new reflexes that influence motor development appear.

TABLE 10-1

GROWTH AND DEVELOPMENT DURING INFANCY

PHYSICAL	GROSS MOTOR	FINE MOTOR	SENSORY	VOCALIZATION	SOCIALIZATION AND COGNITION
Age 1 Month
Weight gain of 150–210 g (5–7 oz) weekly for first 6 months Height gain of 2.5 cm (1 inch) monthly for first 6 months Head circumference increases by 1.5 cm (0.5 inch) monthly for first 6 months Primitive reflexes present and strong Doll’s eye reflex and dance reflex fading Obligatory nose breathing (most infants)	Assumes flexed position with pelvis high but knees not under abdomen when prone (at birth, knees flexed under abdomen)  Can turn head from side to side when prone; lifts head momentarily from bed (see Fig. 10-3, A) Has marked head lag, especially when pulled from lying to sitting position (see Fig. 10-2, A) Holds head momentarily parallel and in midline when suspended in prone position Assumes asymmetric tonic neck flex position when supine When held in standing position, body is limp at knees and hips In sitting position, back is uniformly rounded, with absence of head control	Hands predominantly closed Grasp reflex strong Hand clenches on contact with rattle	Able to fixate on moving object in range of 45 degrees when held at a distance of 20–25 cm (8–10 inches) Visual acuity approaches 20/100* Follows light to midline Quiets when hears a voice	Cries to express displeasure Makes small, throaty sounds Makes comfort sounds during feeding	Is in sensorimotor phase—stage I, use of reflexes (birth–1 month), and stage II, primary circular reactions (1–4 months) Watches parent’s face intently as she or he talks to infant
Age 2 Months
Posterior fontanel closed Crawling reflex disappears	Assumes less flexed position when prone—hips flat, legs extended, arms flexed, head to side Less head lag when pulled to sitting position (see Fig. 10-2, B) Can maintain head in same plane as rest of body when held in ventral suspension When prone, can lift head almost 45 degrees off table When moved to sitting position, head is held up but bends forward (see Fig. 10-5, B) Assumes symmetric tonic neck position intermittently	Hands often open Grasp reflex fading	Binocular fixation and convergence to near objects beginning When supine, follows dangling toy from side to point beyond midline Visually searches to locate sounds Turns head to side when sound is made at level of ear	Vocalizes, distinct from crying Crying becomes differentiated Coos Vocalizes to familiar voice	Demonstrates social smile in response to various stimuli
Age 3 Months
Primitive reflexes fading	Able to hold head more erect when sitting but still bobs forward Has only slight head lag when pulled to sitting position Assumes symmetric body positioning Able to raise head and shoulders from prone position to a 45- to 90-degree angle from table; bears weight on forearms When held in standing position, able to bear slight fraction of weight on legs Regards own hand	Actively holds rattle but will not reach for it Grasp reflex absent Hands kept loosely open Clutches own hand; pulls at blankets and clothes	Follows objects to periphery (180 degrees)  Locates sound by turning head to side and looking in same direction Begins to have ability to coordinate stimuli from various sense organs	Squeals aloud to show pleasure Coos, babbles, chuckles Vocalizes when smiling “Talks” a great deal when spoken to Less crying during periods of wakefulness	Displays considerable interest in surroundings Ceases crying when parent enters room Can recognize familiar faces and objects, such as feeding bottle Shows awareness of strange situations
Age 4 Months
Drooling begins Moro, tonic neck, and rooting reflexes have disappeared	Has almost no head lag when pulled to sitting position (see Fig. 10-2, C)  Balances head well in sitting position (see Fig. 10-5, C) Back less rounded, curved only in lumbar area Able to sit erect if propped up Able to raise head and chest off surface to angle of 90 degrees (see Fig. 10-3, B) Assumes predominant symmetric position  Rolls from back to side	Inspects and plays with hands; pulls clothing or blanket over face in play Tries to reach objects with hand but overshoots Grasps object with both hands Plays with rattle placed in hand and shakes it but cannot pick it up if dropped Can carry objects to mouth	Able to accommodate to near objects Binocular vision fairly well established Can focus on a 1.25-cm (0.5-inch) block Beginning eye–hand coordination	Makes consonant sounds n, k, g, p, b  Laughs aloud Vocalization changes according to mood	Is in stage III, secondary circular reactions Demands attention by fussing; becomes bored if left alone Enjoys social interaction with people Anticipates feeding when sees bottle or mother if breastfeeding Shows excitement with whole body, squeals, breathes heavily Shows interest in strange stimuli Begins to show memory
Age 5 Months
Beginning signs of tooth eruption Birth weight doubles	No head lag when pulled to sitting position When sitting, able to hold head erect and steady Able to sit for longer periods when back is well supported Back straight When prone, assumes symmetric positioning with arms extended  Can turn over from abdomen to back When supine, puts feet to mouth	Able to grasp objects voluntarily Uses palmar grasp, bidextrous approach Plays with toes Takes objects directly to mouth Holds one cube while regarding a second one	Visually pursues a dropped object Is able to sustain visual inspection of an object Can localize sounds made below ear	Squeals Makes cooing vowel sounds interspersed with consonant sounds (e.g., ah-goo)	Smiles at mirror image Pats bottle or breast with both hands More enthusiastically playful but may have rapid mood swings Is able to discriminate strangers from family Vocalizes displeasure when object is taken away Discovers parts of body
Age 6 Months
Growth rate may begin to decline Weight gain of 90–150 g (3–5 oz) weekly for next 6 months Height gain of 1.25 cm (0.5 inch) monthly for next 6 months  Teething may begin with eruption of two lower central incisors  Chewing and biting occur	When prone, can lift chest and upper abdomen off surface, bearing weight on hands (see Fig. 10-3, C) When about to be pulled to a sitting position, lifts head Sits in high chair with back straight Rolls from back to abdomen When held in standing position, bears almost all of weight Hand regard absent	Resecures a dropped object Drops one cube when another is given Grasps and manipulates small objects Holds bottle Grasps feet and pulls to mouth	Adjusts posture to see an object Prefers more complex visual stimuli Can localize sounds made above ear Will turn head to the side and then look up or down	Begins to imitate sounds  Babbling resembles one-syllable utterances—ma, mu, da, di, hi Vocalizes to toys, mirror image Takes pleasure in hearing own sounds (self-reinforcement)	Recognizes parents; begins to fear strangers Holds arms out to be picked up Has definite likes and dislikes Begins to imitate (cough, protrusion of tongue) Excites on hearing footsteps  Briefly searches for a dropped object (object permanence beginning) Frequent mood swings, from crying to laughing, with little or no provocation
Age 7 Months
Eruption of upper central incisors	When supine, spontaneously lifts head off surface  Sits, leaning forward on both hands (see Fig. 10-5, D) When prone, bears weight on one hand Sits erect momentarily Bears full weight on feet (see Fig. 10-6, A) When held in standing position, bounces actively	Transfers objects from one hand to the other (see Fig. 10-5, E) Has unidextrous approach and grasp Holds two cubes more than momentarily Bangs cubes on table Rakes at a small object	Can fixate on very small objects Responds to own name Localizes sound by turning head in a curving arch Beginning awareness of depth and space Has taste preferences	Produces vowel sounds and chained syllables—baba, dada, kaka Vocalizes four distinct vowel sounds “Talks” when others are talking	Increasing fear of strangers; shows signs of fretfulness when parent disappears Imitates simple acts and noises Tries to attract attention by coughing or snorting Plays peek-a-boo Demonstrates dislike of food by keeping lips closed Exhibits oral aggressiveness in biting and mouthing Demonstrates expectation in response to repetition of stimuli
Age 8 Months
Begins to show regular patterns in bladder and bowel elimination Parachute reflex appears (see Fig. 10-4)	Sits steadily unsupported (see Fig. 10-5, E) Readily bears weight on legs when supported; may stand holding onto furniture Adjusts posture to reach an object	Has beginning pincer grasp using index, fourth, and fifth fingers against lower part of thumb Releases objects at will Rings bell purposely Retains two cubes while regarding third cube Secures an object by pulling on a string Reaches persistently for toys out of reach		Makes consonant sounds t, d, w Listens selectively to familiar words Utterances signal emphasis and emotion Combines syllables, such as dada, but does not ascribe meaning to them	Increasing anxiety over loss of parent, particularly mother, and fear of strangers Responds to word “no” Dislikes dressing, diaper change
Age 9 Months
Eruption of upper lateral incisor may begin	Creeps on hands and knees Sits steadily on floor for prolonged time (10 minutes) Recovers balance when leaning forward but cannot do so when leaning sideways  Pulls self to standing position and stands holding on to furniture (see Fig. 10-6, B and C)	Uses thumb and index finger in crude pincer grasp (see Fig. 10-1) Preference for use of dominant hand now evident Grasps third cube Compares two cubes by bringing them together	Localizes sounds by turning head diagonally and directly toward sound Depth perception increasing	Responds to simple verbal commands Comprehends “no-no”	Parent (mother) is increasingly important for own sake Shows increasing interest in pleasing parent Begins to show fears of going to bed and being left alone Puts arms in front of face to avoid having it washed
Age 10 Months
Labyrinth-righting reflex is strongest when infant is in prone or supine position; is able to raise head	Can change from prone to sitting position Stands while holding on to furniture; sits by falling down Recovers balance easily while sitting While standing, lifts one foot to take a step (see Fig. 10-6, D)	Crude release of an object beginning Grasps bell by handle		Says “dada,” “mama” with meaning Comprehends “bye-bye” May say one word (e.g., “hi,” “bye,” “no”)	Inhibits behavior to verbal command of “no-no” or own name Imitates facial expressions; waves bye-bye Extends toy to another person but will not release it  Develops object permanence Repeats actions that attract attention and cause laughter Pulls clothes of another to attract attention Plays interactive games such as pat-a-cake Reacts to adult anger; cries when scolded Demonstrates independence in dressing, feeding, locomotive skills, and testing of parents Looks at and follows picture in a book
Age 11 Months
Eruption of lower lateral incisor may begin	When sitting, pivots to reach toward back to pick up an object  Cruises or walks holding on to furniture or with both hands held	Explores objects more thoroughly (e.g., clapper inside bell) Has neat pincer grasp Drops object deliberately for it to be picked up Puts one object after another into a container (sequential play) Able to manipulate an object to remove it from tight-fitting enclosure		Imitates definite speech sounds	Experiences joy and satisfaction when a task is mastered Reacts to restrictions with frustration Rolls ball to another on request Anticipates body gestures when a familiar nursery rhyme or story is being told (e.g., holds toes and feet in response to “This little piggy went to market”) Plays games up-down, “so big,” or peek-a-boo Shakes head for “no”
Age 12 Months
Birth weight tripled  Birth length increased by 50% Head and chest circumference equal (head circumference 46 cm [18 inches]) Has six to eight deciduous teeth Anterior fontanel almost closed Landau reflex fading Babinski reflex disappears Lumbar curve develops; lordosis evident during walking	Walks with one hand held Cruises well  May attempt to stand alone momentarily; may attempt first step alone Can sit down from standing position without help	Releases cube in cup Attempts to build two-block tower but fails Tries to insert a pellet into a narrow-necked bottle but fails Can turn pages in a book, many at a time	Discriminates simple geometric forms (e.g., circle) Amblyopia may develop with lack of binocularity Can follow rapidly moving object Controls and adjusts response to sound; listens for sound to recur	Says three to five words besides “dada,” “mama” Comprehends meaning of several words (comprehension always precedes verbalization) Recognizes objects by name Imitates animal sounds Understands simple verbal commands (e.g., “Give it to me,” “Show me your eyes”)	Shows emotions such as jealousy, affection (may hug or kiss on request), anger, fear Enjoys familiar surroundings and explores away from parent Is fearful in strange situation; clings to parent May develop habit of “security blanket” or favorite toy Has increasing determination to practice locomotor skills  Searches for an object even if it has not been hidden but searches only where object was last seen

Milestones that represent essential integrative aspects of development that lay the foundation for the achievement of more advanced skills.

^*Degree of visual acuity varies according to vision measurement procedure used.

The chest assumes a more adult contour, with the lateral diameter becoming larger than the anteroposterior diameter. The chest circumference approximately equals the head circumference by the end of the first year. The heart grows less rapidly than does the rest of the body. Its weight is usually doubled by 1 year of age in comparison with body weight, which triples during the same period. The size of the heart is still large in relation to the chest cavity; its width is approximately 55% of the chest width.

It is important to note that genetic, metabolic, environmental, and nutritional factors strongly influence infant growth; thus, the previous statements are general guidelines only. Use the appropriate infant growth charts reflecting weight for length and head circumference in each case to determine appropriate growth parameters. The World Health Organization growth charts released in 2006 are now recommended as reference growth charts in children 0 to 59 months of age (Grummer-Strawn, Reinold, Krebs, and others, 2010). (See World Health Organization [WHO] growth charts in Appendix A).

Maturation of Systems

Other organ systems also change and grow during infancy. The respiratory rate slows somewhat (see inside back cover) and is relatively stable. Respiratory movements continue to be abdominal. Several factors predispose infants to more severe and acute respiratory problems than older children. The close proximity of the trachea to the bronchi and its branching structures rapidly transmits infectious agents from one anatomic location to another. The short, straight eustachian tube closely communicates with the ear, allowing infection to ascend from the pharynx to the middle ear. In addition, the inability of the immune system to produce immunoglobulin A (IgA) in the mucosal lining provides less protection against infection in infancy than during later childhood.

The heart rate slows (see inside back cover), and the rhythm is often sinus arrhythmia (rate increases with inspiration and decreases with expiration). Blood pressure also changes during infancy (see Appendix E). Systolic pressure rises during the first 2 months as a result of the increasing ability of the left ventricle to pump blood into the systemic circulation. Diastolic pressure decreases during the first 3 months and then gradually rises to values close to those at birth. Fluctuations in blood pressure occur during varying states of activity and emotion.

Significant hematopoietic changes occur during the first year of life (see Appendix B). Fetal hemoglobin (HgbF) is present for the first 5 months, with adult hemoglobin steadily increasing through the first half of infancy. Fetal hemoglobin results in a shortened survival of red blood cells (RBCs) and thus a decreased number of RBCs. A common result at 2 to 3 months of age is physiologic anemia. High levels of fetal hemoglobin are thought to depress the production of erythropoietin, a hormone released by the kidneys that stimulates RBC production.

Maternally derived iron stores are present for the first 5 to 6 months of life and gradually diminish, which also accounts for lowered hemoglobin levels toward the end of the first 6 months. The occurrence of physiologic anemia is not affected by an adequate supply of iron. However, when erythropoiesis is stimulated, iron supplies are necessary for the formation of hemoglobin.

The digestive processes are relatively immature at birth. Although term newborn infants have some limitations in digestive function, human milk has properties that partially compensate for decreased digestive enzymatic activity, thus enabling breastfed infants to receive optimal nutrition during the first several months of life. Saliva is secreted in small amounts, but the majority of the digestive processes do not begin functioning until age 3 months, when drooling is common because of the poorly coordinated swallowing reflex. The enzyme amylase (also called ptyalin) is present in small amounts but usually has little effect on the foodstuffs because of the small amount of time the food stays in the mouth. Gastric digestion in the stomach consists primarily of the action of hydrochloric acid and rennin, an enzyme that acts specifically on the casein in milk to cause the formation of curds—coagulated semisolid particles of milk. The curds cause the milk to be retained in the stomach long enough for digestion to occur.

Digestion also takes place in the duodenum, where pancreatic enzymes and bile begin to break down protein and fat. Secretion of the pancreatic enzyme amylase, which is needed for digestion of complex carbohydrates, is deficient until about the fourth to sixth month of life. Lipase is also limited, and infants do not achieve adult levels of fat absorption until 4 to 5 months of age. Trypsin is secreted in sufficient quantities to catabolize protein into polypeptides and some amino acids.

The immaturity of the digestive processes is evident in the appearance of stools. During infancy, solid foods (e.g., peas, carrots, corn, raisins) are passed incompletely broken down in the feces. An excess quantity of fiber easily disposes infants to loose, bulky stools.

During infancy, the stomach enlarges to accommodate a greater volume of food. By the end of the first year, infants are able to tolerate three meals a day and an evening bottle and may have one or two bowel movements daily. However, with any type of gastric irritation, infants are vulnerable to diarrhea, vomiting, and dehydration (see Chapter 24).

The liver is the most immature of all the gastrointestinal organs throughout infancy. The ability to conjugate bilirubin and secrete bile is achieved after the first couple of weeks of life. However, the capacities for gluconeogenesis, formation of plasma protein and ketones, storage of vitamins, and deaminization of amino acids remain relatively immature for the first year of life.

Maturation of the suckling, sucking, and swallowing reflexes and the eruption of teeth (see Teething, p. 325) parallel the changes in the gastrointestinal tract and prepare infants for the introduction of solid foods.

The immunologic system undergoes numerous changes during the first year. Full-term newborns receive significant amounts of maternal immunoglobulin G (IgG), which, for approximately 3 months, confers immunity against antigens to which their mothers were exposed. During this time, infants begin to synthesize IgG; approximately 40% of adult levels are reached by 1 year of age. Significant amounts of immunoglobulin M (IgM) are produced at birth, and adult levels are reached by 9 months of age. Secretory IgA is not present at birth but is found in saliva and tears by 2 to 5 weeks. Prebiotic oligosaccharides found in breast milk produce probiotic bacteria such as bifidobacteria and lactobacilli, which in turn stimulate synthesis and secretion of sIgA. Secretory IgA is present in large amounts in colostrum; IgA confers protection to the mucous membranes of the gastrointestinal tract (Blackburn, 2011; Lawrence and Lawrence, 2011) against many bacteria, such as Escherichia coli, and viruses such as rubella, poliovirus, and the enteroviruses. The development of the mucosa-associated lymphoid tissue occurs during infancy; in part, this system is believed to prevent colonization and passage of bacteria across the infant’s mucosal barrier (Lawrence and Lawrence, 2011). The function and quantity of T lymphocytes, lymphokines, interferon-γ, interleukins, tumor necrosis factor-α, and complement are reduced in early infancy, thus preventing optimal response to certain bacteria and viruses. The production of IgA and immunoglobulins D and E (IgD and IgE) is much more gradual, and maximum levels are not attained until early childhood. Probiotics may have a significant role in helping the gastrointestinal tract establish a “good” bacterial colonization in the gut to prevent many illnesses, including antibiotic-induced diarrhea and possibly Helicobacter pylori gastritis (Thomas, Greer, American Academy of Pediatrics [AAP], and others, 2010).

Evidence indicates that vernix caseosa, a white oily substance that coats term infants’ bodies and is often found in abundance in creases of the axilla and groin, has innate immunologic properties that serve to protect newborns from infection (Narendran and Hoath, 2006). Vernix also appears to have a role in maintaining the integrity of the stratum corneum and facilitating acid mantle development (Hoath, Pickens, and Visscher, 2006). The epidermis of a full-term infant undergoes maturation during the first month of life; the newborn’s skin acts as a barrier to infection, assists in thermal regulation, and prevents transepidermal water loss in term infants.

During infancy, thermoregulation becomes more efficient; the ability of the skin to contract and of muscles to shiver in response to cold increases. The peripheral capillaries respond to changes in ambient temperature to regulate heat loss. The capillaries constrict in response to cold, conserving core body temperature and decreasing potential evaporative heat loss from the skin surface. The capillaries dilate in response to heat, decreasing internal body temperature through evaporation, conduction, and convection. Shivering (thermogenesis) causes the muscles and muscle fibers to contract, generating metabolic heat, which is distributed throughout the body. Increased adipose tissue during the first 6 months insulates the body against heat loss.

A shift in the total body fluid occurs; at birth, 75% of a term infant’s body weight is water, and there is an abundance of extracellular fluid (ECF). As the percentage of body water decreases, so does the amount of ECF—from 40% at term to 20% in adulthood. The high proportion of ECF, which is composed of blood plasma, interstitial fluid, and lymph, predisposes the infant to a more rapid loss of total body fluid and, consequently, dehydration. The loss of 5% to 10% of term newborns’ initial birth weight in the first 5 days of life is attributed to ECF compartment contraction, enhanced renal tubular function, and rapidly increasing glomerular filtration rate (Blackburn, 2011).

The immaturity of the renal structures also predisposes infants to dehydration and electrolyte imbalance. Complete maturity of the kidneys occurs during the latter half of the second year, when the cuboidal epithelium of the glomeruli becomes flattened. Before this time, the filtration capacity of the glomeruli is reduced. Urine is voided frequently and has a low specific gravity (1.000–1.010). At term, most infants produce and excrete approximately 15 to 60 ml/kg/24 hr, and an output of less than 0.5 ml/kg/hr after 48 hours of age is considered to be oliguria (Blackburn, 2011).

Auditory acuity is at adult levels during infancy. Visual acuity begins to improve, and binocular fixation is established. Binocularity, or the fixation of two ocular images into one cerebral picture (fusion), begins to develop by 6 weeks of age and should be well established by age 4 months. Depth perception (stereopsis) begins to develop by age 7 to 9 months but may not be fully mature until 2 or 3 years of age, thus increasing infants’ and younger toddlers’ risk of falling.