There are several reasons why some couples need donated eggs in order to have the best chance, or any chance of having a family.
Some women may be born without ovaries or with ovaries but without eggs inside them. Women are born with all of the eggs they will ever have and there is no treatment to create or multiply them. The eggs are aging with the woman and their quality deteriorates with time. By the time of the menopause, when the menstrual cycle stops, all the eggs have been used up. Most of the eggs available in the ovary do not actually ovulate over a lifetime. They are lost with the day to day turnover of cells like hair and skin. For some women the ovaries age prematurely. This can be as early as the teenage years or in one’s 20’s, long before parenthood is seriously considered for many people. This is called premature ovarian insufficiency.
Some women and girls will lose their eggs or whole ovaries as a result of surgery or cancer treatments including chemotherapy and radiotherapy. Unfortunately, unlike hair which regrows, new eggs do not develop.
The most common situation where egg donation is considered is in women whose own ovaries are too weak to try IVF or who have had unsuccessful IVF largely because of the poor quality of their own eggs. This is more common in women over 40 years old and can result in repeatedly unsuccessful treatments or miscarriages.
There are some rare conditions that are inherited through the female line via the egg genes.
Some couples may choose to avoid the risk of having a child with a serious disability who may suffer handicap and a short life expectancy, by using an egg from a donor who does not carry the condition in her genes.