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Before birth, physical development lays the foundation for future behavior and mental processes. Key stages, beginning with genetic formation at conception to the development of the brain and nervous system, influence how infants will respond to the world after birth. The time between conception and birth, referred to as prenatal development, shapes sensory abilities, reflexes, and early learning capacity, all which impact behavior and cognitive processing after birth. Understanding prenatal development helps psychologists connect the physical changes that occur in the womb to observable behaviors and mental abilities throughout life.
Adverse Factors Affecting Fetal Development
Although the womb provides protection for the developing fetus, several types of factors can cause fetal harm. These factors include teratogens, maternal illness, genetic mutations, hormonal factors, and environmental conditions.
Teratogens: Teratogens are substances that can cause birth defects or developmental issues when an embryo or fetus is exposed to them. A wide range of illnesses, drugs, and toxins, collectively considered teratogens, can harm the fetus. Common teratogens include alcohol, tobacco, certain medications, and illicit drugs.
- Alcohol: Mothers who drink heavily during pregnancy may have babies with fetal alcohol syndrome, characterized by problems such as small head size, heart defects, irritability, hyperactivity, mental retardation, or slowed motor development.
- Tobacco: Maternal smoking is associated with lower birth weights, higher risk of premature birth, and higher risk of cleft lip and cleft palate.
- Certain prescription or over-the-counter drugs: The antibiotic streptomycin can cause fetal deafness. Aspirin taken after 20 weeks increases the risk of fetal kidney problems.
- Recreational drugs: Children born to mothers who use methamphetamine, for example, are more likely to have heart and brain abnormalities.
It should be noted that teratogens can also include exposures described in the following sections. For instance, maternal infections such as rubella, syphilis, or Zika virus, can cross the placenta and affect the fetus. Additionally, certain environmental exposures such as radiation exposure and chemicals like lead, mercury, and pesticides are also considered teratogens.
Maternal illness: Some maternal illnesses can cross the placental barrier and affect the fetus, especially if the mother contracts certain infections early in pregnancy. For example, rubella, toxoplasmosis, and Zika virus are linked to serious congenital abnormalities, including vision or hearing impairments, neurological complications, and growth restrictions. Other infections, such as HIV/AIDS, syphilis, listeriosis (a foodborne illness), and severe influenza, can also pose risks to fetal health, potentially leading to developmental issues or birth defects.
Genetic Mutations: Genetic mutations, whether inherited or occurring spontaneously during early cell divisions, can lead to various developmental disorders or conditions. Down Syndrome, caused by an extra copy of chromosome 21, and cystic fibrosis, a result of a mutation on chromosome 7, are examples of genetic factors that alter both physical development and, in some cases, cognitive functioning.
Hormonal Factors: Hormones play a crucial role in directing the growth and differentiation of cells in the fetus. Imbalances or disruptions in maternal or fetal hormone levels can affect organ development, brain structure, and even behavior. For instance, exposure to high levels of stress hormones in the mother can impact fetal stress responses and may influence behavioral tendencies later in life. Even mild maternal hypothyroidism, for instance, can affect fetal brain development.
Environmental Factors: Environmental conditions, such as maternal nutrition, access to healthcare, and exposure to pollution, can significantly impact prenatal development. Malnutrition can hinder brain and body growth, while exposure to pollutants or toxins may increase the risk of developmental issues, particularly in respiratory and immune system functions.
- Environmental toxins: Lead, once commonly found in gasoline and paint, damages the fetal nervous system. Organic solvents such as trichloroethylene (TCE) in drinking water can cause a range of birth defects.
- X-rays and other kinds of radiation: In high doses, radiation can cause small head size and can affect the fetus’ bones, eyes or genitals. With the dosages encountered in medical settings, however, the risk is small.
You may notice that environmental factors are comprised of some, but not all, teratogens. The term teratogens is a more specific term referring to particular agents or substances that have been shown to cause developmental harm to an embryo or fetus, such as drugs, alcohol, infectious diseases, or certain chemicals. Teratogens are usually identified based on direct evidence of harmful effects on prenatal development. Environmental factors refer to a broader scope of factors that can include both teratogenic agents and other elements of the prenatal environment, such as nutrition, stress, pollution, or lifestyle factors. While some environmental factors, such as exposure to pesticides, can act as teratogens, others, like maternal stress, might influence fetal development indirectly or have more varied effects on psychological and physical development.
Infancy and Childhood
Many of newborns’ abilities are innate, that is, present from birth. From day one, infants possess motor reflexes such as the rooting reflex (moving the mouth side to side, feeling for the mother’s nipple) and the grasping reflex (clenching the fingers when the palm is stroked). These automatic responses help infants interact with their environment and support essential functions for survival. For instance, the rooting reflex helps with feeding and demonstrates early sensory processing and motor coordination. Other reflexes, such as the grasping reflex and the Moro reflex (an instinctive startle response), help demonstrate neurological functioning. These reflexes are key developmental milestones, as they provide evidence that an infant’s nervous system and sensory systems are developing on track. Gradually, as infants grow, they begin to replace reflexive movements with more voluntary, controlled actions, signaling progress toward complex motor and cognitive skills.
Newborns also have sensory abilities; they can hear, smell, touch, taste, and see. Even depth perception has been demonstrated in infants, by their reluctance to crawl across a “visual cliff” formed by a clear plexiglass sheet. The “visual cliff” experiment is a landmark study in developmental psychology that demonstrates that infants possess an early capacity for depth perception. Researchers Eleanor Gibson and Richard Walk (1960) used an apparatus called the visual cliff – a platform with a shallow surface on one side and a deep-looking “drop” on the other, covered by sturdy, clear glass. Infants were placed on the shallow side, with their caregivers calling them from across the apparent “drop.”
Results indicated that many infants were hesitant to crawl over the “cliff,” suggesting that they could perceive the depth and sense potential danger. Most infants as young as six-months old would hesitate or refuse to crawl over the visual cliff. However, some evidence suggests that younger infants, even as young as two to three months old, show physiological responses (like changes in heart rate) when placed on the deep side, suggesting an even earlier sensitivity to depth that is present before they begin crawling.
Motor Development
Motor development also progresses quickly. Motor development is the increasing coordination of muscles that makes physical movements possible. Developmental norms tell us the median age at which babies develop specific behaviors and abilities. Babies often deviate a fair amount from these norms, but the order in which skills are acquired rarely varies much. Motor skills are divided into one of two categories: gross motor skills and fine motor skills. Gross motor skills involve larger muscle groups, such as those needed for sitting, crawling, walking, and running. Fine motor skills rely on smaller muscles for tasks like grasping objects, drawing, and, eventually, writing.
Gross motor skills develop first, laying the foundation for movement and coordination. For instance, infants begin by learning to support their heads, then progress to rolling over, sitting up, crawling, and eventually walking. “Tummy time” is often recommended for young infants to encourage the development of these essential gross motor skills. Fine motor skills progress from basic hand-eye coordination, like reaching for objects, to more precise movements, like stacking blocks or using utensils. Offering infants “finger foods,” such as peas or Cheerios, is a way to help them practice and develop their fine motor skills.
Motor skill development is partly a function simply of maturation, genetically programmed growth and development as the child ages. However, recent research suggests that motor development isn’t just a passive process. Babies also actively develop motor skills by moving around and exploring their environments. Finally, cultural factors matter. For example, in cultures where babies receive early training in sitting up, standing, and walking, they develop these skills earlier. Conversely, in cultures where mothers carry babies most of the time, babies develop these skills later.
Critical and Sensitive Periods of Development
In early childhood, there are critical or sensitive periods when infants and young children are especially primed for learning certain skills. This is true for language acquisition and, later, for musical training. For instance, in the early years of life, children are very responsive to language exposure, which helps in the development of grammar, vocabulary, and pronunciation. Limited language input during this critical period may result in permanent gaps in language ability, which is why learning new languages is far easier at a young age but becomes much more difficult later in life.
In some nonhuman animals this concept can be seen in imprinting. Imprinting is a survival mechanism observed in species such as ducks and geese, where the newborns instinctively attach and follow the first moving object they see, typically their mother. This attachment ensures the young animal stays close to a caregiver for safety and nourishment. Imprinting and critical periods show how early exposure to specific stimuli can have lasting effects on behavior and development. Understanding critical periods in human development helps psychologists, educators, and caregivers ensure that children are provided with the appropriate stimuli and support at the appropriate key developmental stages.
Physical Changes in Adolescence
Adolescence is marked by significant physical changes as individuals transition from childhood to adulthood. These changes are driven primarily by hormonal shifts, leading to rapid growth and numerous changes that signal the body’s preparation for reproductive capabilities. Pubescence refers to the two years before puberty. The adolescent growth spurt begins during pubescence, at about age 11 in girls and about age 13 in boys. At this time, children get taller and heavier, their primary sex characteristics mature, and they develop secondary sex characteristics.
Primary sex characteristics refer to reproductive organs, that is the body structures directly involved in reproduction. These include the reproductive organs and genitalia that are essential for sexual reproduction. In females, primary sex characteristics include the ovaries, fallopian tubes, uterus, vagina, and vulva. In males, primary sex characteristics include the testes, seminal vesicles, prostate gland, and penis.
Secondary sex characteristics are sex-specific physical characteristics that are not essential for reproduction. Girls develop breasts, widened pelvic bones, wider hips, and increased body fat in areas like the thighs and buttocks. Boys develop facial hair, broader shoulders, increased muscle mass, and deeper voices. Both sexes experience growth of underarm and pubic hair, changes in body composition, and skin changes, such as increased oil production.
After pubescence and at the beginning of adolescence, puberty occurs. Puberty is the point at which sexual organs mature and reproductive ability develops. Sexual organs include the ovaries in girls and the penis and testes in boys.
Menarche, or the first menstrual period, marks the onset of puberty in girls. The average age of menarche for American girls is about 12.5 years old. The beginning of nocturnal emissions, so-called wet dreams, marks the onset of puberty in boys. Girls reach full sexual maturation around age 16, and boys reach sexual maturity at around 18.
Spermarche is the beginning of sperm production, marking the onset of reproductive capability in males. It typically occurs during puberty, around the ages of 12 to 14, although exact ages can vary. Spermarche is considered a significant developmental milestone in adolescence, similar to menarche in females, and it indicates the body’s readiness for reproductive functions.
Earlier Onset of Puberty
Girls and boys in the United States reach puberty earlier now than they did a few generations ago, possibly because nutrition and medical care have changed over the years. In Western Europe and the United States, girls have their first menstrual periods at around age 12 or 13. In poorer regions of Africa, which lack proper nutrition and health care, girls may not begin to menstruate until they are between the ages of 14 and 17.
Varying Maturation Rates
Puberty occurs at different rates for different people. In girls, puberty usually happens between ages 10 and 15 and in boys between ages 11 and 16. Early-maturing girls and late-maturing boys tend to have more psychological and social problems than their peers. In girls, a correlation exists between early maturation and poorer school performance, earlier sexual activity, more unwanted pregnancies, and a higher likelihood of eating disorders. Both boys and girls who mature early use more alcohol and drugs and have more problems with the law than their peers.
Physical Changes in Adulthood
Adulthood is the longest stage of human development, marked initially by stability and eventually by gradual physical decline. During early and middle adulthood, many physical abilities, such as strength, sensory acuity, and reaction time, remain relatively stable. However, aging brings various changes:
Reproductive changes: As adults age, reproductive ability begins to decrease. In females, this culminates in menopause, which typically occurs in the late 40s or early 50s. Menopause is the cessation of menstruation and marks the end of reproductive capability. In males, there is often a gradual decline in testosterone levels, though men generally retain fertility throughout their lives.
Mobility and flexibility: Muscular strength, flexibility, and joint mobility tend to decline gradually in the early-30s to mid-30s, affecting physical endurance and agility. Initial changes are often subtle and go unnoticed, with more noticeable declines occurring for many adults around middle age (40s to 50s) as muscle mass and joint elasticity decrease. Many adults experience decreased stamina and a slower recovery time after physical exertion. Regular physical activity, particularly stretching and strength training, can slow down this decline and help maintain mobility and flexibility longer into adulthood.
Reaction time: Reaction times tend to slow in older adulthood. While early adulthood often involves peak reaction times, tasks that require quick responses, like driving, become more challenging as adults age.
Sensory decline: Vision and hearing abilities also decline with age. For instance, many adults develop presbyopia (difficulty focusing on close objects), and they may need reading glasses. Hearing loss, particularly for high-pitch sounds, becomes common as well, a condition known as presbycusis.
