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.
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.
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:
|infancy||0 - walking|
|toddlerhood||walking - 3|
|preadolescence||10 - 12|
|adolescence||13 - 19|
Note: the Tanner stages can be used to approximately judge a child's age based on physical development.