Physical Growth in the First Two Years
Infants grow dramatically from birth to age two. Height typically increases from about 45‑55 cm at birth to roughly 91 cm by the second birthday. Weight also doubles during the first year, rising from an initial range of 5‑10 livres to about 22 livres. Rapid growth spurts can add as much as 1.25 cm in a single 24‑hour period. Before a spurt, children often become more irritable, hungry, and need extra sleep.
Body composition changes as well. Body fat reaches its peak around nine months of age. After this point, fat gradually declines as muscle mass increases.
Brain Development and Maturation
The newborn brain contains roughly 100 to 200 billion neurons. During the first two years, these neurons form countless synaptic connections, creating the foundations for cognition and behavior. While many connections are pruned, the elimination rate is about 40‑60 %, not the previously claimed 90 %.
Glial cells, which make up about half of brain tissue, are essential for myelination, allowing faster signal transmission. Electroencephalography (EEG) studies use 128 electrodes in early childhood and increase to 256 electrodes later, reflecting the growing complexity of neural activity.
Functional magnetic resonance imaging (fMRI) is challenging for children under five to six years old because it requires the child to remain still for extended periods.
The cerebral cortex accounts for approximately 85 % of total brain weight. Developmental timelines differ across regions: the visual cortex matures primarily in the first year, whereas language‑related cortex develops during the preschool years. The left hemisphere is dominant for language and pleasant emotions, while the right hemisphere supports spatial abilities and less pleasant emotions.
Bone age, assessed by X‑ray, remains the most reliable indicator of physical maturity. Girls typically achieve higher bone‑age maturity than boys throughout childhood, offering some protective advantage against adverse environments.
Growth Patterns Across Populations
Average body size varies by ancestry: Asian children are generally smaller than European children, who in turn are smaller than African‑American children.
Sleep Needs and Patterns
Newborns sleep 16‑18 hours per day, distributed across multiple bouts. By two years, sleep needs decrease to 12‑13 hours per 24‑hour period. Napping usually ceases between three and five years as the brain matures.
Nutrition and Breastfeeding
Human breast milk contains more fat and less protein than formula, supporting myelination and overall brain growth. In developing countries, breastfed infants have a survival rate 6 to 14 times higher than formula‑fed infants.
Co‑sleeping Practices and Safety
Co‑sleeping is common worldwide, with about 90 % of parents sharing a bed with their infant. In contrast, only about 20 % of parents in the United States practice co‑sleeping. Safe sleeping arrangements recommend that infants sleep in the parents’ room (not the same bed) during the first year.
Risk factors for unsafe co‑sleeping include obesity, alcohol consumption, and excessively soft mattresses. These factors increase the likelihood of accidental suffocation or overheating.
Motor and Sensory Milestones
Motor development follows two complementary patterns: cephalocaudal development proceeds from head to feet, while proximodistal development proceeds from the trunk outward. Gross motor milestones involve large‑scale movements such as crawling and walking, whereas fine motor milestones involve precise hand actions like the pincer grasp.
Infants begin to perceive colors similarly to adults around four months of age. Word recognition emerges near ten months, paving the way for rapid vocabulary expansion by age two.
Depth perception experiments demonstrate that infants avoid crossing a plexiglass barrier when the gap is too deep, indicating early spatial awareness.
Sensitive Periods and Plasticity
‘Sensitive periods’ are windows during which specific experiences critically shape neural circuits. Deprivation during these periods can cause permanent, irreversible damage. Contrary to some beliefs, children’s brains exhibit greater plasticity than adult brains after injury, leading to fewer long‑term sequelae.
