Development: is the sequence of age-related changes that occur as a person progresses from conception to death. Zygote: a one-celled organism formed by the union of a sperm and an egg. Prenatal period: extends from conception to birth, usually encompassing nine months of pregnancy. First stage of prenatal development, encompassing the first 2 weeks after conception. Zygote becomes a mass cells, on 7th day it implants its self to the uterine wall. Placenta: provides oxygen and nutrients from the mother to the fetus. Embryonic stage: is the second stage of development, lasting from 2 weeks until the end of the second month. Vital organs are developing in this stage. Embryo starting to look like human, arms, legs, fingers, toes, eyes. Most structural defects occur/develop during this stage. Fetal stage: third stage of prenatal development, lasting from 2 months through birth. Sex organs start to develop during 3rd month.