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Biology LibreTexts

23.5: Infancy

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    22629
  • What a Life!

    Sleep, cry, eat, repeat. Oh, the life of an infant. Newborn infants actually do spend most of their time in these three “pursuits.” However, by the end of their first year, they have greatly expanded their repertoire. By their first birthday, infants are typically starting to walk and talk, and they are spending about as much time awake as asleep. Clearly, infancy is a time of tremendous change.

    baby sleeping with pasifier
    Figure \(\PageIndex{1}\): Image used with permission (CC BY-NC 2.0; Fimb; flickr.com)

    Defining Infancy

    Infancy refers to the first year of life after birth, and an infant is defined as a human being between birth and the first birthday. The term baby is usually considered synonymous with an infant, although it is commonly applied to the young of other animals, as well as humans. Human infants seem weak and helpless at birth, but they are actually born with a surprising range of abilities. Most of their senses are quite well developed, and they can also communicate their needs by crying, like the three-day-old baby in Figure \(\PageIndex{2}\). During their first year, infants develop many other abilities, some of which are described in this concept. They also grow more rapidly during their first year than they will at any other time during the rest of their life.

    Human Male White Newborn Baby Crying
    Figure \(\PageIndex{2}\): just a few days old, this baby already knows how to communicate his needs to his caregivers by crying. (Public domain; Evan-Amos; via wikimedia.org)

    Neonate

    A newborn infant is called a neonate up until the first four weeks after birth. A neonate, like the crying baby pictured above, does not usually look like the plump, chubby-cheeked “Gerber baby” that most people envision when they hear the term "baby."

    Status of the Newborn: Apgar Score

    Immediately after birth, a simple test called an Apgar test, is administered to an infant to evaluate its transition from the uterus to the outside world (Figure \(\PageIndex{3}\)). The newborn is assessed on each of five easy-to-measure traits, and for each trait is given a score of 0, 1, or 2 (where 0 is the worst value and 2 is the best). After an infant is assessed on each trait, the values of all five traits are added together to yield the Apgar score. The highest (best) possible score is 10, but a score of 7 or higher is considered normal. A score of 4-6 is considered fairly low, and a score of 3 or lower is considered critically low. The purpose of the Apgar test is to determine quickly whether a newborn needs immediate medical care. It is not designed to predict long-term health issues.

    New Born with vernix
    Figure \(\PageIndex{3}\): This newborn infant, with vernix still coating her forehead and shoulders, is being given an Apgar test right after birth. The stethoscope on her chest is being used to detect and measure her heart rate. This is one of five traits that will be assessed for her Apgar score. (public domain; Bigroger27509; via Wikimedia.org)

    The five traits that are assessed in an Apgar test are listed in Table \(\PageIndex{1}\). The table also shows how the acronym APGAR can be used to help remember the five traits.

    Table \(\PageIndex{1}\): Apgar Test

    Acronym (APGAR)

    Trait

    Score of 0

    Score of 1

    Score of 2

    A = Appearance

    skin color

    blue or pale all over

    blue at extremities; body pink

    extremities and body both pink

    P = Pulse

    heart rate

    absent

    <100 beats per minute

    >100 beats per minute

    G = Grimace

    reflex irritability grimace

    no response to stimulation

    grimace on suction or aggressive stimulation

    cry on stimulation

    A = Activity

    activity

    none

    some flexion

    flexed arms and legs that resist extension

    R = Respiration

    respiratory effort

    absent

    weak, irregular gasping

    strong, robust cry

    Umbilical Cord

    The umbilical cord of a newborn infant contains the umbilical arteries and vein. The cord will normally be cut within seconds of birth, leaving a stub about 3-5 cm (1-2 in.) long (Figure \(\PageIndex{4}\)). The umbilical stub will dry out, shrivel, darken, and spontaneously fall off within about three weeks of birth. This will become the navel after it fully heals.

    umbilical cord being cut

    Figure \(\PageIndex{4}\): The umbilical cord is cut right after birth, leaving a short stub that will wither away and form the navel within a month. (Public Domain; Mech via Wikimedia.org).

    Physical Characteristics of the Neonate

    Right after birth, a newborn’s skin is wet. It may be covered with streaks of blood, and coated with patches of waxy white vernix. The newborn may also have peeling skin on the wrists, hands, ankles, and feet. Some newborns still have the fine, colorless hair called lanugo, but it usually disappears within the first month after birth. Infants may be born with a full head of hair, or they may have very little hair, or even be bald. A newborn’s body proportions are distinctive, as well. The shoulders and hips are relatively wide, and the arms and legs are relatively long compared to the rest of the body. In addition, the abdomen protrudes slightly.

    Male figures showing proportions in five ages.
    Figure \(\PageIndex{5}\): By drawing the human figure at various ages (from birth to adulthood) all the same size, the differences in body proportions are easier to see. (CC BY-NC 3.0; American Genetic Association)

    A newborn’s head, especially the cranium, is very large in proportion to the body. As shown Figure \(\PageIndex{5}\), the newborn head makes up about one-quarter of the baby’s total body length, whereas the head of an adult makes up only about one-seventh of the adult’s total body length. The body is drawn to be the same length (height) at each age to make the differences in body proportion — and especially head size — more apparent.

    Many regions of the neonate’s skull have not yet been converted to bone, leaving “soft spots” known as fontanels (Figure \(\PageIndex{6}\)). The two largest fontanels are the diamond-shaped anterior (frontal) fontanel, located at the top front of the skull, and the smaller triangular-shaped posterior (occipital) fontanel, located at the back of the head. During birth, the fontanels enable the bony plates of the skull to move and change shape, allowing the child's head to pass through the birth canal. Right after birth, the head may be temporarily misshapen for this reason. The ossification of the bones of the skull causes the posterior fontanel to close during the first two or three months after birth, and the anterior fontanel to close by nine to 18 months after birth.

    neonate’s skull
    Figure \(\PageIndex{6}\): The neonate’s skull viewed from the top shows the frontal (anterior) fontanel (above the face) and the occipital (posterior) fontanel (toward the back of the head). (CC BY-NC 3.0; Henry Vandyke Carter)

    Size and Growth of the Neonate

    In the wealthier nations of the world, the total body length of a full-term infant at birth normally ranges between 46 and 56 cm (18 and 22 in.), with an average of 51 cm (20 in.). Birthweight of a full-term infant normally ranges from 2.5 to 4.5 kg (5.5 to 10 lb), with an average of 3.4 kg (7.5 lb). For pre-term infants, these numbers are likely to be lower, because these infants have had a shorter period of prenatal growth.

    During the first week following birth, it is normal for the weight of a neonate to decrease by about three to seven percent of the birth weight. For example, a baby born at an average weight of 3.4 kg (7.5 lb) might weigh only 3.2 kg (7.1 lb) by the seventh day after birth. This loss of weight is a normal result of the resorption and urination of the fluid that initially fills the lungs. A contributing factor may be a delay of a few days before feeding becomes well established, which is also normal. After the first week, a healthy neonate should start to gain up to 20 g (0.7 oz) per day.

    Neonate Senses

    Some senses in newborns are already relatively well developed. Other senses are still immature and need to develop further after birth.

    Sense of Touch in the Neonate

    Newborns have a well-developed sense of touch, and they usually respond positively to soft stroking and cuddling. Gentle rocking back and forth, massages, and warm baths are also positively received by neonates, and they may calm a crying infant. Newborns can often comfort themselves by sucking their thumb, finger, or a pacifier.

     

    Vision in the Neonate

    Newborns' vision is not yet fully developed. Both the retinas and the parts of the brain involved in vision are still immature. Most newborns are only able to focus on objects that are directly in front of their face and about 46 cm (18 in.) away. However, this is sufficient for the infant to see the mother’s face, as well as the areola and nipple of the breast. When a newborn is not feeding, sleeping, or crying, it is generally staring at objects within its visual range. Usually, anything that is shiny, has sharp contrasting colors, or has a complex pattern will catch an infant’s eye. However, the neonate, like the infant pictured below, has a clear preference for looking at human faces above all else.

    Baby breastfeeding
    Figure \(\PageIndex{7}\): The eyes of this breastfeeding neonate are intently focused on the mother’s face. (CC BY-NC 3.0; USDA)

    Newborns have limited color perception. About three-quarters of newborns can distinguish red, but fewer than half can distinguish green, yellow, or blue. Color perception, however, improves quickly after birth. A newborn infant also lacks depth perception, which is the ability to see in three dimensions. This ability starts to develop only after an infant becomes mobile later in infancy. It continues to develop throughout early childhood.

    Hearing in the Neonate

    A sense of hearing is well developed at birth. Newborns usually respond more readily to female than male voices, and the sound of voices, especially the mother’s voice, may have a soothing effect on the infant. Sounds that the infant heard before birth — such as the mother’s breathing and heartbeat — are also comforting to the newborn.

    Loud or sudden noises, on the other hand, are likely to startle and frighten a newborn. The neonate also responds to sounds of potential danger — such as angry voices of adults, thunder, or the cries of other infants — with greater attention. They may turn toward the sounds and blink their eyes.

    Taste and Smell in the Neonate

    Newborns can respond to different tastes, including sweet, sour, bitter, and salty tastes. They generally show a preference for sweet tastes. They also show a preference for the smell of foods that their mother ate regularly during pregnancy. Presumably, this occurs because amniotic fluid changes taste with different foods eaten by the mother.

    Newborn Reflexes

    Newborns have a number of instinctive behaviors, or reflexes, that help them survive. Crying is one example. It is instinctive in newborn infants, who may use it to express a variety of feelings, such as hunger, discomfort, overstimulation, or loneliness. The need to suckle is also instinctive. They have a sucking reflex that allows them to extract milk from the mother’s nipple or from the nipple on a bottle right after birth. In addition, infants have an instinctive behavior known as the rooting reflex that helps them find the nipple by touch. When an infant’s cheek is stroked or it rubs against an object, the baby automatically turns its head in that direction to find the nipple.

    Infants are born with other reflexes that aid them in maintaining close physical contact with their mother or another caregiver. These reflexes help them hold onto the caregiver so they are less likely to fall, and also so they can satisfy their basic need for constant physical contact. Two of these reflexes are the Moro reflex and the grasping reflex.

    • The Moro reflex is an instinctive behavior that is normally present in an infant from birth up until about three or four months of age. It occurs in response to a sudden loss of support when the infant feels as though it is falling. It involves three distinct components: suddenly spreading out the arms, bringing the arms back in toward the body, and, usually, crying. If the baby really were falling, these motions might help it reach out and grab its mother or another caregiver.
    • The grasping reflex is the instinctive grasping of a finger or other object that is placed in the palm of an infant. This reflex actually arises before birth and is present until an infant is about five or six months of age. It may help an infant grip and hang on to the mother or another caregiver.
    Adult baby bedtime
    Figure \(\PageIndex{8}\): The grasping reflex in this four-month-old infant gives it a surprisingly strong grip. This reflex is already present at birth. (CC0; Wayne Evans via wikimedia.org)

    Milestones in Infant Development

    Many developments occur during infancy. These include developments in several areas — motor skills, sensory abilities, and cognitive abilities. Infants vary in the exact timing of these developments, but the sequence of the developments is usually similar from one infant to another.

    Two Months

    During the first two months after birth, an infant normally develops the ability to hold her head erect and steady when she is held in an upright position. She will also develop the ability to roll from her side to her back. She is likely to start cooing and babbling at her parents and other people she knows, and she will also start smiling at her parents (Figure \(\PageIndex{9}\)).

    Happy Baby
    Figure \(\PageIndex{9}\): A baby’s first smile is an early milestone in infant development. (CC BY-NC 3.0; George Keith; via flickr.com)
     

    Four Months

    By the end of the fourth month after birth, an infant can roll from front to side, lift his head 90 degrees while lying prone, sit up with support, and hold his head steady for brief periods. He will turn his head toward sounds and follow objects with his eyes. He will start to make vowel sounds and begin to laugh. He may even squeal with delight.

    Six Months

    Around six months of age, an infant is normally able to pick up objects and transfer them from hand to hand. She can also pull herself into a sitting position. Her vision will have improved so it is now almost as acute as adult vision. The infant will also start noticing colors and start to show the ability to discriminate depth. She is likely to enjoy vocal play and may start making two-syllable sounds such as “mama” or “dada.” She may also start to show anxiety toward strangers.

    Ten Months

    10 months old baby crawling
    Figure \(\PageIndex{10}\): This ten–month old infant has just learned to crawl. (Mattes; public domain; via Wikimedia.org)

    By about ten months of age, an infant can wiggle and crawl, like the infant pictured below, and can sit unsupported. If she drops a toy she will look for it, and she can now pick up objects with a pincer grasp (using the tips of the thumb and forefinger). She babbles in a way that starts to resemble the cadences of speech. She is likely to exhibit fear around strangers.

    Twelve Months

    By the end of the first year, an infant normally can stand while holding onto furniture or someone’s hand. He may even be starting to walk, as the infant in the photo below. When he drops toys, he watches where they go. He may cooperate with dressing, and he may wave goodbye. He may also babble a few words repeatedly and show that he understands simple commands.

    one year old learning to walk by pushing wheeled toy
    Figure \(\PageIndex{11}\): Using a push toy for balance and support allows this one-year-old to take a few steps. (CC BY-NC 2.0; Shaun MItchem; via Wikimedia.org)

    Dental Development in the First Year

    The deciduous (baby) teeth generally start to emerge around six months of age. The emergence of teeth is called teething. While the teeth are close to emerging through the gums, the gums may become red, swollen, and painful. The baby is likely to drool and be fussy during the few days it takes for the teeth to finally emerge. The baby might also refuse to eat or drink because of the discomfort. The two lower central incisors usually emerge first at about six months, followed by the two upper central incisors at about eight months. The four lateral incisors (two upper and two lower) emerge at roughly ten months.

    Physical Growth in the First Year

    Human height growth per month chart, United States
    Figure \(\PageIndex{12}\): The 50th percentile for the rate of increase (mm/month) in height is shown in this graph separately for males (blue) and females (red) from birth to age 20 years. The 50th percentile is the height below which 50 percent of individuals in a population fall. The graph clearly shows that growth is more rapid during the first year of life than it is at any other time. (Cantus; public domain; via Wikimedia.org)

    Infancy is the period of most rapid growth after birth. Growth during infancy is even faster than it is during puberty when the adolescent growth spurt occurs, as shown in the following graph.

    Growth in Weight and Length

    Following the initial weight loss right after birth, an infant normally gains an average of about 28 g (1 oz) per day during the first two months. Then, weight gain slows somewhat, and the infant normally gains about 0.45 kg (1 lb) per month during the remainder of the first year. At this rate of weight gain, an infant generally doubles its birth weight by six months after birth and triples its birth weight by 12 months after birth.

    Growth in overall body length is also very rapid during infancy, especially in the first few months. Infants normally grow about 2.5 cm (1.0 in.) per month during the first six months. During the second six months, they normally grow about 1.2 cm (0.5 in.) per month. At this rate of growth in length, an infant may close to double its birth length by the end of the first year!

    During doctor visits throughout the first year of life, a baby’s weight and length are measured. The baby’s values are compared to standard weight and length values for infants of the same age to assess whether the baby is growing normally. The actual weight and length are generally considered to be less important than evidence showing that the baby is failing to grow normally between visits. Babies that grow too slowly may have a health problem or may be undernourished. If this goes uncorrected, it can produce permanent deficits in size. On the other hand, a faster-than-normal increase in weight may result in the infant becoming too heavy and being at greater risk of obesity later in life.

     

    Variations in Growth

    Not all body parts and organs grow at the same rate during infancy. The head and trunk normally show the greatest growth. This allows for rapid growth and development of the brain, heart, and lungs. At birth, the baby’s brain is already about one-quarter of its future adult size, but it normally increases to about three-quarters of its adult size by the end of the first year. The kidneys also develop rapidly during infancy, so by the end of the first year, they are functioning at an adult level. The reproductive system goes through a short period of rapid growth soon after birth but thereafter grows very slowly until puberty.

    Infant Mortality

    Of all stages in life, infancy is associated with a higher risk of death than any other stage, except old age. Worldwide, the major causes of death in infancy include dehydration, infection, birth defects, and sudden infant death syndrome (SIDS). Deaths that occur during the first year of life are referred to as infant mortality. At a population level, infant mortality is measured by the infant mortality rate, which is defined as the number of infant deaths per one thousand live births.

    The infant mortality rate is regarded as a very important measure of the level of health in a nation. The level of health, in turn, usually reflects a nation’s wealth, because wealth is associated with access to medical care, socioeconomic conditions that promote health, and public health practices. Not surprisingly, there is a well-documented inverse relationship between national wealth and the infant mortality rate. The graph below shows this relationship clearly. In the graph, wealth is measured as the gross domestic product (GDP) per person (per capita), as expressed in U.S. dollars. As the graph shows, the highest levels of infant mortality occur in the poorest countries, and as wealth rises, the infant mortality rate falls sharply.

    Infant mortality vs GDP
    Figure \(\PageIndex{13}\): The infant mortality rate is a good measure of the health effects of wealth — or lack thereof — in a population. (Public domain; Donaharr2 via Wikicommons).

    Feature: Reliable Sources

    Sudden infant death syndrome (SIDS) is the unexplained death, usually during sleep, of a seemingly healthy infant. In the U.S., SIDS is one of the leading causes of death in the first year of life, with about two thousand infants dying in the U.S. each year from SIDS. The cause of SIDS is unknown, although scientists suspect that immaturity or abnormality of the part of the brain that controls arousal from sleep and breathing may be involved. Researchers have also identified several factors that increase the risk of SIDS. Some risks include male sex, pre-term birth, low birth weight, exposure to secondhand smoke, and sleeping on the stomach. Certain practices — such as placing an infant on its back to sleep and not using pillows or blankets in the crib — can help reduce the risk of SIDS.

    Go online to learn more about SIDS. Find reliable sources that answer the following questions.

    1. What current research is being undertaken to better understand the cause of SIDS? What risk factors or areas of concern are being investigated?
    2. How can parents reduce the risk of SIDS in their infants? Which three reliable sources of information on SIDS would you recommend to new parents to raise their awareness of SIDS and how to reduce the risk of SIDS in their infants?

    Summary

    • Infancy refers to the first year of life after birth. An infant is defined as a human being between birth and the first birthday. A newborn baby is called a neonate up until the first four weeks after birth.
    • Immediately after birth, an Apgar test is administered to an infant to determine whether the newborn needs urgent medical care. The baby is scored on five traits, including skin color and heart rate. The umbilical cord is also cut within seconds of birth, leaving a stub that will eventually dry out and fall off, forming the naval.
    • Newborns may or may not have vernix or lanugo covering the skin, and they may or may not have head hair. Their body proportions are distinctive, and the head is very large relative to the body. Soft spots in the skull called fontanels — which allow the head to change shape slightly to fit through the birth canal — gradually ossify after birth.
    • A well-nourished, full-term newborn averages about 51 cm (20 in.) in length and has an average birth weight of 3.4 kg (7.5 lb). A newborn typically loses a small amount of weight in the first week, but after that, a healthy neonate should start gaining weight rapidly.
    • Newborns have well-developed senses of touch and hearing, and they can respond to different tastes and smells. However, their sense of vision is not yet fully developed. Their visual acuity is poor, and they also have limited color and depth perception.
    • Infants are born with several reflexes that help them survive the first few months of life: crying for communication, suckling, and the rooting reflex (which helps them find a nipple). The Moro and grasping reflexes help them maintain close physical contact with the mother or another caregiver.
    • Many important developments in motor, sensory, and cognitive abilities occur during infancy. There is variation among infants in the exact timing of these developments, but the sequence in which they occur is usually similar.
    • The deciduous teeth generally start to emerge around six months of age. This is called teething, and it may cause discomfort and fussiness. Typically, all of the upper and lower incisors emerge during the first year.
    • Infancy is the period of most rapid growth after birth. A healthy, well-nourished infant generally triples his birth weight and doubles his birth length by the first birthday. The head and trunk normally grow most rapidly, allowing rapid growth and development of the brain, heart, and lungs.
    • Infancy is associated with a higher risk of death than any other life stage, except old age. The infant mortality rate — defined as the number of infant deaths per one thousand live births — is an important measure of the level of health in a nation. It tends to be inversely correlated with a nation’s wealth.

    Review

    1. Define infancy, infant, and neonate.

    2. What is an Apgar test? When and why is it administered?

    3. Describe what happens to the umbilical cord after birth.

    4. What are some physical characteristics of a neonate?

    5. What are the average length and weight of a well-nourished, full-term newborn?

    6. Why do newborns typically lose some weight in the first week after birth?

    7. Describe newborn sensory abilities.

    8. Identify some of the reflexes that are present in newborn infants, and how they help the newborn survive.

    9. Identify a milestone in infant development that typically occurs by each of the ages below. In general, how does the timing of developmental milestones vary among infants at the ages of: two month, four months, six months, ten months, and one year

    10. Outline dental development in the first year.

    11. Describe growth during infancy.

    12. Define the infant mortality rate, and explain its significance.

    13. A mother brings her six-month-old to visit the pediatrician. She is concerned that he does not weigh nearly as much as his cousin, who is the same age. What is one piece of information that the pediatrician would likely want to know in order to help assess whether the infant’s weight is a concern?

    14. A baby is born and a nurse immediately records the observations below. What is this baby’s APGAR score? Is this score considered normal? Explain your answer.

    • Its skin is blue at the extremities, but the body is pink.
    • Its heart rate is 98 beats per minute.
    • Baby cries on stimulation.
    • Baby has flexed arms and legs that resist extension.
    • Baby has a strong, robust cry.

    Explore More

    Blood in the umbilical cord is a rich source of stem cells that could potentially cure diseases, but cord blood is usually disposed of after birth. Watch the video below to learn about a public cord blood banking program that stores donated cord blood so that these valuable stem cells can be used to save lives.

    Watch the video below to see some newborn reflexes in action: