The first stage in human development occurs when a female oocyte merges with male spermatozoon in a process known as fertilization or conception. This can take place during sexual intercourse or as a result of artificial insemination. After fertilization the joined oocyte and spermatozoon are referred to as an egg, which develops and divides over a period of nine months into an infant ready for birth.
When a baby is born it is cared for and fed by its parents. In the first year of life a child will develop teeth, learn to walk and talk and grow significantly larger. Rates of development depend on the individual child, but during babyhood children are completely dependent on their parents.
As a child develops from a baby into a young child it continues to grow in size and develop increased motor skills and dexterity. A child will loose its baby teeth and grow a set of permanent adult teeth. At age five a human child is able to walk, run, talk and feed itself but is still largely dependent on his parents.
Around the age of 12, depending on the particular child, adolescence begins in human children leading to sexual maturity. Female children gain the ability to give birth to their own offspring and develop mammary glands or breasts to nurse infants. During female adolescence the menstrual cycle also begins. Male children begin to produce sperm in their testes or testicles and alongside females will grow and develop to their adult proportions.
When a child reaches the end of adolescence they are considered adults. They will not grow anymore and will not experience any more major physical changes. Adult humans become independent from their parents and begin to produce their own children. As humans live beyond adolescence they begin to age and deteriorate before eventually dying. The World Bank estimates the life expectancy of a human in the United States to be 78.4 years.