Prenatal and Infant Physical Development
The prenatal period of development begins with conception and ends at birth (approximately nine months later). The fertilized egg, called a zygote, is made up of genetic material (46 chromosomes grouped into 23 pairs). Half of the genetic material comes from the mother's egg and half from the father's sperm. Eggs can contribute X chromosomes, and sperm can contribute either X or Y chromosomes. If the zygote contains two X chromosomes, then it is genetically female. If it contains an X and a Y chromosome, it is genetically male. The zygote divides to form a ball of cells, called a blastocyst, which is made up of an inner group of cells with an outer shell. The inner group of cells develops into an embryo and then into a fetus. It takes about three to four days for the blastocyte to reach the uterus. It takes another two weeks to implant into the uterine wall.
A developing baby is called an embryo through the eighth week of gestation (pregnancy). One month into a pregnancy, the embryo is about the size of a pea. At two months, it has grown to the size of a kidney bean. By the end of the second month, the heart, lungs, brain, spinal cord, arms, and legs have formed. After the eighth week of pregnancy and until birth occurs, a developing baby is called a fetus.
Human pregnancies are divided into three trimesters, each lasting about three months. The first trimester is the period when most miscarriages and birth defects occur. Threats to typical development include genetic disorders, prematurity, low birth weight, and exposure to toxins. Harmful substances that can impair development of an embryo or fetus are called teratogens. Alcohol consumption during pregnancy can impact brain development, resulting in a set of cognitive and emotional processing defects called fetal alcohol spectrum disorders. According to research, babies born to women who smoke during pregnancy tend to weigh less than normal and are more likely to show perceptual and attentional problems in childhood.
Once a fetus is born, it is called an infant. Human infants are born with a head that is disproportionately large. Over the course of development, the length of limbs relative to the torso increases and the relative size of the head decreases accordingly. Infants are born with specific reflexes that help ensure survival. These include grasping, sucking, and rooting (turning the face toward an item that is touching the cheek and making sucking motions). Fine and gross motor skills develop in a predictable order. Motor skills emerge in sequence from the head to the feet and from the center of the body outward. Infants first learn to lift their heads at around one month of age and push their upper bodies up with their arms when lying on their stomachs at around three months. Next, infants learn to roll over at around four months and sit up at four to nine months. Older infants learn to crawl at 5 to 14 months and generally begin to cruise, or move sideways while holding on to furniture for support, at around 10 months. Infants typically learn to stand alone between 7 to 17 months and begin to walk alone in the range of 8 to 18 months.
Toddler and Preadolescent Physical Development
From the toddler years through the elementary school years, children improve their strength, balance, and coordination. Girls typically develop fine motor skills (movements involving smaller muscles in the hands and feet), such as drawing or stringing beads, more quickly than boys. Boys typically develop gross motor skills (movement and coordination of larger body parts like the arms and legs), such as climbing or jumping, more quickly than girls. This may have more to do with the activities boys and girls practice than with innate biological differences.Around eight to nine years of age, the brain experiences a growth spurt. It increases to nearly the same size as an adult brain. As different parts of the brain develop, such as the frontal lobes, children's cognitive capacity increases. They become able to carry out increasingly complex tasks. They can follow several steps to achieve an end goal, such as assembling a toy piece by piece.
Stages of Motor Development
Adolescent Physical Development
Adolescence begins with the onset of sexual maturity. During the transition from childhood to adolescence, adolescents gain about 40 pounds on average and grow approximately 10 inches. This transition occurs around 11 to 14 years of age, with girls showing physical changes at younger ages than boys. Adolescence ends with the beginning of adulthood. In the United States, people legally become adults at age 18; however, cultural shifts mean many people do not have the education or financial resources to live independently of their families at that age. Adolescents frequently attend college rather than entering the workforce directly after high school. Although they may live outside the home while at college, they are often still somewhat dependent on their families for financial and emotional support. Significant cognitive, social, and emotional development continues past 18 years of age. The brain is not fully developed until around age 25. Many individuals also begin to examine their future more seriously (in terms of possible careers, places to live, and relationships) later in adolescence.
The onset of sexual maturity is called puberty, marked by dramatic bodily changes and an intensification in sexual interest and desire. These developments are driven by changes in primary and secondary sex characteristics. Primary sex characteristics refer to bodily structures directly involved in reproduction. Boys become fertile as they experience enlargement of the testes, scrotum, and penis and develop the ability to ejaculate. Once this process is complete, they are continually fertile. Girls achieve menarche the first time they menstruate. Menstruation is a sign that a female has become fertile (able to become pregnant). Menstruation occurs in monthly cycles, driven by increases and decreases in levels of the hormones estrogen and progesterone. In most species of mammals, females are receptive to sex only when they are fertile (estrus). Human females are sexually receptive throughout their cycle, but their sexual desire often peaks around ovulation when they are most fertile.Secondary sex characteristics are physical traits that change with sexual maturity but have no direct link to reproduction. For males and females, these changes include growth of armpit and pubic hair and deepening of the voice. Breast enlargement occurs for girls, growth of facial hair occurs for boys. Growth does not occur evenly across the body. Typically hands and feet grow first, then arms and legs, then the torso. Females typically reach their full adult height by age 14, whereas males reach their full adult height around age 16. Both males and females continue to gain weight and muscle after they have reached their full height.
Changes in Body Proportions Over Time
Adult Physical Development
Adults reach their peak levels of strength, agility, stamina, and vigor during their twenties. By the age of 30, each of these begins to decline. Older adults typically show declines in balance, coordination, and reaction time. The rate of physical changes varies greatly across individuals. Ongoing physical activity and balance training can help people maintain strength and mobility. Regular exercise also helps to prevent disease and reduce the risk for dementia.
Fertility in males and females declines rapidly after age 35. Women may remain fertile until about age 50 when their estrogen levels drop steeply and they undergo menopause, or the cessation of menstrual periods. Although men remain fertile longer than women, sperm from men over age 40 is linked to lower rates of conception and higher miscarriage rates. Older men are also more likely to father children with genetic disorders due to age-related fragmentation of sperm DNA.
Nearly all current theories of aging attribute aging to the accumulation of molecular damage. Some theories, such as programmed aging theory, assert that this waste buildup is encoded in the human genome, a kind of programmed self-destruction. Others claim that the cause of aging is molecular damage itself (wear and tear on tissues). Some of this damage is attributed to changes in mitochondria, which control how cells in the body "breathe" and produce energy. Cumulative mutations (genetic changes) in mitochondria over time can result in aging.