A stage is usually defined by an expected age range and its accompanying identifiable developmental milestones. Below is a common classification of age ranges. Adolescence 20 years to 40 years Young Adulthood 40 years to 65 years Middle Adulthood 65 years and up LateAdulthood Biological Development Prenatal development is characterized by rapid changes. It is broken down into stages. We commonly hear of the three trimesters that mark the equal division of the nine months of pregnancy.
Prenatal development is also usefully divided into the germinal, embryonic and fetal stages.
This growing mass of cells travels through the mother's fallopian tube to the uterus. The placenta surrounds and protects the zygote and brings in nutrients and oxygen while removing wastes. At approximately two weeks after conception, the zygote has increased in mass and become implanted in the uterus. At this point we begin to speak of it as the embryo. The embryonic stage lasts from approximately week two to week eight.