At birth a woman has a finite number of oocytes in her ovaries, which is around 1-2 million oocytes. However, this number will have already been reduced to around 500,000 oocytes by puberty.
Subsequently, as menstrual cycles progress, a woman's ovarian reserve (the number of oocytes in her ovaries at any given time) is progressively reduced until it is depleted at menopause, which is the end of menstrual cycles.
Age is therefore a very important factor in the decrease in ovarian reserve, although it is not the only one. Thus, the gynaecologist can assess a woman's ovarian reserve with methods such as the antral follicle count (AFC).