Human development (biology)

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
Development begins with fertilization, the process by which the male gamete, the sperm cell, and the female gamete, the egg, fuse to produce a zygote. In pregnancy the 3 stages are commonly referred as Z.E.F. - meaning Zygote, Embryo, Fetus.

In medicine, the beginning of pregnancy is the instant a sperm cell enters an ovum and forms a viable zygote. Recently, in western medicine, pregnancy is defined as beginning when a 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 an unborn child helps get the nutrients to the child and helps get rid of the waste from the child. Before the placenta is developed the blastocyst receives its nutrients from the yolk sac, which is contained within the blastocyst.

The zygote undergoes rapid cell 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. Age is defined relative to this event in most cultures.

Physical stages of human life
There are no universal definitions for terms of age-related physical development stages, but following are some approximate age ranges:


 * Prenatal (fertilization - birth)
 * Embryo - (fertilization - 8 weeks of gestational phase)
 * Zygote, the point of conception, fertilization
 * Blastocyst the period between conception and embryonic stages
 * Embryo; the embryonic period starts at three weeks and continues until the end of the 8th week of pregnancy
 * Fetus (8 weeks of gestational phase - birth)


 * Child (Childbirth)(0 - 12)
 * Neonate (newborn) (0 – 30 days)
 * Infant (baby) (1 month - 12 months)
 * Toddler (1 – 3 years)
 * Play age (4–5 years)
 * Primary school age (also called prepubescence) (4-12)
 * Elementary school age (also called middle childhood) (4-9)
 * Preadolescence (preteen, or late childhood. The child in this and the previous phase are called schoolchild (schoolboy or schoolgirl), when still of primary school age.) (10 – 12 years)
 * Adolescence and puberty (13 – 19 years)
 * Peripuberty (8 -10 until 15 -17 )
 * Adult (20+ years)
 * Early adulthood (20 – 39 years)
 * Middle adulthood (40 – 59 years)
 * Advanced adulthood/Senior citizen (60+ years)
 * Death (occurs at various ages, depending on person)
 * Decomposition (breakdown of the body after death)

Also sometimes used are terms that specify one's age in numbers, such as:
 * Child (0-12)
 * Teenager (13-19)
 * Twentysomething (20-29)
 * Thirtysomething (30-39)
 * Fortysomething (40-49) (formerly also Quadragenarian, rarely used since 1980)
 * Quinquagenarian (50-59)
 * Sexagenarian (60-69)
 * Septuagenarian (70-79)
 * Octogenarian (80-89)
 * Nonagenarian (90-99)
 * Centenarian (100-109)
 * Supercentenarian (110+)

Physical development milestones

 * Ability to lift and control the orientation of the head
 * Crawling begins
 * Walking begins
 * Speech begins
 * Voice lowers in pitch (especially noticeable in boys)
 * Pubic hair appears
 * Genitals and reproductive organs mature
 * Menstrual cycle begins (females)
 * Body hair and facial hair appears

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