According to WebMD, females have around 1 million eggs at birth. By the time a female reaches puberty, she only has approximately 300,000 eggs. Out of all of these eggs, only 300 to 400 ever ovulate.
WebMD says that the majority of eggs within a woman’s ovaries steadily die on their own. This deterioration is unaffected by external factors, so a woman's egg supply continues to decrease whether or not she has a regular menstrual cycle. Even when a woman is pregnant or on birth control, her number of viable eggs decreases constantly. Once a woman reaches menopause, her eggs are completely depleted.