Human Development · 4 min read
Prenatal Development · Infancy and Toddlerhood
Prenatal development occurs in three periods. The germinal period spans conception through the second week. During this time, the fertilized egg (zygote) undergoes rapid cell division and travels to the uterus for implantation. The embryonic period spans weeks 3 through 8. Major organs and body systems begin to form. The embryo is highly vulnerable to teratogens during this time. The fetal period spans week 9 through birth. Organs and systems continue to develop and mature. The fetus becomes capable of survival outside the womb around 22 to 26 weeks.
Teratogens are agents that can cause birth defects or developmental abnormalities. The effects depend on the timing, dose, and duration of exposure, as well as genetic susceptibility. Alcohol can cause fetal alcohol spectrum disorders, characterized by facial abnormalities, growth deficits, and cognitive impairments. Tobacco increases risk of low birth weight, preterm birth, and sudden infant death syndrome. Illicit drugs can cause withdrawal symptoms, developmental delays, and behavioral problems. Certain medications, including some prescription drugs, can cause birth defects. Environmental hazards such as lead, mercury, and radiation can affect prenatal development. Maternal infections including rubella, cytomegalovirus, and toxoplasmosis can cause serious harm.
Maternal factors affecting prenatal development include nutrition, age, stress, and health conditions. Adequate folic acid intake prevents neural tube defects. Advanced maternal age increases risk of chromosomal abnormalities. Chronic stress can affect fetal development through hormonal mechanisms. Conditions such as diabetes and hypertension require careful management during pregnancy.
Genetic abnormalities include chromosomal disorders and single-gene disorders. Down syndrome results from an extra chromosome 21. Turner syndrome affects females with only one X chromosome. Klinefelter syndrome affects males with an extra X chromosome. Single-gene disorders include phenylketonuria, sickle cell disease, cystic fibrosis, and Huntington's disease.
Prenatal testing and screening options include ultrasound, maternal blood tests, amniocentesis, and chorionic villus sampling. These procedures can identify many genetic abnormalities and structural defects before birth.
Physical development during infancy is rapid and follows cephalocaudal (head to toe) and proximodistal (center to extremities) patterns. Birth weight typically doubles by 5 months and triples by 12 months. Brain development is extensive during the first two years. Synaptic connections proliferate and are pruned based on experience.
Motor development follows a predictable sequence, though timing varies among individuals. Reflexes present at birth include rooting, sucking, grasping, and stepping. Gross motor milestones include head control, rolling over, sitting, crawling, pulling to stand, and walking. Fine motor development progresses from reflexive grasping to voluntary reaching and manipulation.
| Age | Gross motor | Fine motor | Language and social |
|---|---|---|---|
| 2 months | Holds head up during tummy time | Opens hands briefly | Coos; social smile |
| 6 months | Rolls both ways; sits with support | Rakes small objects; passes toy hand to hand | Babbles; laughs; knows familiar faces |
| 9 months | Sits without support; crawls | Pincer grasp begins | Stranger anxiety; plays peekaboo |
| 12 months | Pulls to stand; cruises; may take first steps | Releases objects into a container | First words such as mama; waves bye-bye |
| 18 months | Walks well; climbs | Scribbles; stacks 2–4 blocks | Says several words; points to show interest |
| 2 years | Runs; kicks a ball | Turns pages; stacks about 6 blocks | Two-word phrases; parallel play |
| 3 years | Pedals a tricycle; climbs stairs alternating feet | Copies a circle | Three- to four-word sentences; takes turns |
| 5 years | Hops; skips | Uses scissors; prints some letters | Tells a simple story; cooperative play |
Sensory development is largely complete at birth, though perceptual abilities continue to develop. Visual acuity improves rapidly during the first few months. Depth perception develops around 6 to 8 months. Hearing is well-developed at birth. Infants show preferences for human faces and voices.
Cognitive development during infancy involves sensorimotor learning. Object permanence develops gradually between 8 and 12 months. Infants demonstrate deferred imitation and symbolic thinking by the end of this period.
Language development begins with cooing around 2 months and babbling around 6 months. First words typically appear around 12 months. Vocabulary expands rapidly during the second year. Two-word combinations emerge around 18 to 24 months.
Socioemotional development includes formation of attachment relationships. Stranger anxiety and separation anxiety appear around 8 to 12 months. Self-awareness emerges around 18 months, demonstrated by mirror self-recognition. Emotional regulation begins to develop with caregiver support.
Temperament refers to individual differences in reactivity and self-regulation. Thomas and Chess identified three temperament types: easy, difficult, and slow-to-warm-up. Temperament has biological bases but is shaped by experience.
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