Final answer:
Infancy is characterized by the significant physical, cognitive, and sensory development that occurs during the first-year post-birth, where infants triple their weight and double their length, and achieve various developmental milestones.
Step-by-step explanation:
Infancy is the phase of rapid growth and development that occurs in the first year of life after birth. During this period, an infant will experience significant physical, motor, sensory, and cognitive changes. Notably, a healthy infant will likely triple in birth weight and double in birth length, and undergo development of the brain, heart, and lungs due to rapid growth of the head and trunk. Infancy includes various developmental milestones such as teething, which generally begins around six months. Infants start life with certain abilities such as well-developed senses of touch and hearing, but vision is not fully developed at birth. They utilize reflexes like suckling and rooting to find a nipple and maintain close contact with caregivers, signaling their basic needs predominantly through crying.
Motor skills' development, sensory perception enhancement, and cognitive ability progress are also significant during this stage. Developments occur in a sequence that is similar among infants, although the timing may vary individually. These stages set the foundation for the more complex stages of development that follow as the infant transitions into early childhood.