Human Body

Reproduction and Human Development

What are the stages of postnatal development?

The term “Siamese twins” originated with the appearance of Chang and Eng Bunker (1811–1874), conjoined Chinese twins born in Siam (now Thailand), who were used as a circus attraction by P.T. Barnum (1810–1891).

Siamese twins are identical twins joined at some point of their bodies, most commonly at the hip, chest, abdomen, buttocks, or head. Like other identical twins, they originate from a single fertilized egg; in the case of congenitally joined twins, however, the egg fails to split into two separate cell masses at the proper time. The condition is relatively rare; only about 500 cases have been reported worldwide. Surgery to separate Siamese twins is a complex task, and often results in the death of one or both of the twins.

The five life stages of postnatal development are: 1) neonatal, 2) infancy, 3) childhood, 4) adolescence, and 5) maturity. The neonatal period extends from birth to one month. Infancy begins at one month and continues to two years of age. Childhood begins at two years of age and lasts until adolescence. Adolescence begins at around 12 or 13 years of age and ends with the beginning of adulthood. Adulthood, or maturity, includes the years between ages 18 to 25 and old age. The process of aging is called senescence.


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