Human development (biology)

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]


Human development is the process of growing to maturity. In biological terms, this entails growth from a one-celled zygote to an adult human being.

Biological development

A spermatozoon fertilizing an ovum

Development begins with fertilization, the process by which the male gamete, the sperm cell, and the female gamete, the oocyte, fuse to give rise to a diploid cell, the zygote. In pregnancy the 3 stages are commonly referred to as ZEF - meaning Zygote, Embryo, Fetus.

In medicine, pregnancy is defined as beginning when a fertilized zygote becomes implanted in a woman's uterus. This occurs when the zygote then becomes embedded into the endometrium (lining of the uterus) where it forms a placenta, for the purpose of receiving essential nutrients through the uterus wall. The umbilical cord in a newborn child signifies the remnants of implantation.

The zygote undergoes rapid mitotic divisions with no significant growth (a process known as cleavage) and cellular differentiation, leading to development of an embryo.

Childbirth is the process in which the baby is born. It is considered by many to be the beginning of a person's life, where age is defined relative to this event in most cultures.

Physical stages

Terms for stages of age-related physical development include, with their approximate age ranges:

Also sometimes used are terms that specify one's age in numbers, such as:

Comparison table

Child prenatal >0
infancy 0 - walking
toddlerhood walking - 3
play age 3-6
preadolescence 10 - puberty
adolescence puberty - 19
Young adult twentysomething 20-29
thirtysomething 30-39
Middle age fortysomething 40-49
quinquagenarian 50-59
Advanced adult sexagenarian 60-69
septuagenarian 70-79
octogenarian 80-89
nonagenarian 90-99
centenarian 100-109
supercentenarian 110

Physical development milestones

Note: the Tanner stages can be used to approximately judge a child's age based on physical development.

See also


  1. Approximately



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