Fertility relates to the number of eggs you have. Women are born with all the eggs that they are going to have; no new eggs are made in a woman’s lifetime. It varies from individual-to-individual, but a woman is born, with around two million eggs.
Over the course of time, the number of eggs will reduce; this is a natural process and not a cause for concern.
By the time she reaches puberty and begins menstruation (the start of having periods), the number of eggs you have has fallen to somewhere between 300,000 and 500,000. At around 35 years of age, the decline in the number of available eggs becomes more rapid, until menopause – which typically happens in a woman’s 50s – when the supply of eggs ceases.