At each of the two ends of the reproductive lifespan, after the onset of puberty and prior to menopause, menstrual cycle intervals may be irregular and generally longer in duration.
A woman has an estimated 300 to 400 thousand eggs contained within her ovaries at the time of puberty. She is usually presumed to have no remaining eggs at menopause, although research has recently demonstrated that she may actually have a few hundred eggs remaining at menopause. Since a normal reproductive lifespan typically covers the ages 14 to 44 there are about 30 years of ovulation with about 12 ovulations per year for a total of about 300 to 400 ovulations. Thus, a woman "uses up" an average of about 1000 eggs per menstrual cycle (300 to 400 thousand eggs in 300 to 400 ovulations).
It is not clear whether these eggs are used at a uniform rate over the entire reproductive years or whether there is a certain time period of rapid loss. It does appear that about 10-15 years prior to menopause (at about the age 37-38 and continuing for up to a few years) there is a period of accelerated loss of follicles with an initial shortening of the egg maturing phase of the menstrual cycle (follicular phase). This period of accelerated loss does not fully account for the enormous loss of eggs over the reproductive lifespan.
A follicle is a cyst in the ovary containing an egg. Most likely, of the 1000 eggs that are committed for any given cycle only a small number of follicles (maybe 5-25) actually begin the process of maturation at the onset of the cycle.