Ovarian aging. You were born with all the eggs you will have. Each month, a number start out and only 1 ovulates. If you are on birth control pills or are pregnant, the same thing happens but you won't ovulate. Therefore the number continues to decline. If you have ovarian surgery, endometriosis, or smoke, that may affect the number and quality of the remaining eggs. Some women will run out earlier than others.
They decline. Egg quality and quantity typically decline as a woman ages.