The human lifespan has been many different stages. These stages are identified as; conception, pregnancy (gestation), birth and infancy, childhood, adolescence, adulthood, older adulthood and final stages of life. The age range of some of these life stages are defined by social criteria.
The age range for the life stages are:
A human life begins with conception. Conception is the time that the child was being conceived. A fertile woman usually produces one egg cell each month, roughly two weeks after the last menstrual cycle. If sexual intercourse takes place while the egg is in the fallopian tube, there is a possibility of conception as the egg is getting ready to become a baby, but if there is no sperm then after the respiration has happened the egg will leave the body. Conception is when the sperm reaches and fertilise the egg which is what helps you discover how long you have been pregnant. It is difficult for someone to find out the exact day that the egg was fertilised as the sperm can stay in the body for days before it actually reaches the egg and fertilise it.
Pregnancy begins when a sperm penetrates an egg. One to one and a half days later, the single fertilised egg cell begins to divide. After two or three days there are enough new cells to make the fertilised egg the same size as a pin head. The collection of cells travels to the lining of the uterus where it becomes anchored. The developing collection of cells is now called an embryo which is attached the wall of the uterus by the placenta.
Once the embryo is attached to the uterus wall a chemical signal stops the woman from having another menstrual period. After a period of eight weeks the embryo may have grown to between 3 and 4cm and have a recognisable heartbeat and the beginning of eyes, ears, a mouth, legs and arms. At this stage the growing organism is called a foetus.