While it's normal for fertility to decrease in women over 40, any falloff of fertility prior to that may indicate a condition known as premature ovarian failure (POF). Woman suffering from POF (one in 1,000 by age 30) have ovaries that don't function normally and fail to produce eggs [source: National Institutes of Health].
Although doctors don't know the exact causes of the condition, it's evident that it can occur in one of two ways. In the case of follicle depletion, there are no viable follicles left in the ovaries. Follicles are the part of the reproductive system that release eggs, and if they're all damaged, this critical part of the fertility process can't happen.
Follicle dysfunction is the other way in which POF can manifest, and in this case, there are still viable follicles in the ovaries. They just don't function correctly.
As one of the main symptoms of POF is an irregular menstrual cycle, a common treatment for the condition is hormone replacement therapy to provide the woman with the hormones she needs. However, no treatment exists that can restore normal ovarian function, although fertility drugs can help a small percentage of women. So those afflicted with POF often turn to alternative ways of conceiving, such as the use of donor eggs.