The ovaries are parts of the female reproductive system. They are about the size and shape of an almond and are found just above the fallopian tubes - one ovary on each side of the uterus. Every month during ovulation, either the right or left ovary produces a single mature egg for fertilization. Each ovarian follicle contains a hollow ball of cells with an immature egg in the center. During childhood, approximately half of the ovarian follicles are absorbed by the body. By the time a girl reached puberty and her menstrual cycle begins, only about 400,000 ovarian follicles are left to develop into mature eggs. A baby girl is born with about 1,000,000 ovarian follicles. The number of follicles in the ovary decreases throughout childhood and adult life. By menopause the ovaries are almost bereft of follicles.
Ovarian follicle is the basic unit of female reproductive biology and is made up of roughly spherical collection of cells found in the ovary. They contain a single oocyte or egg. These follicles are periodically stimulated to grow and develop, resulting in ovulation of usually a single competent oocyte. These eggs are only developed once every menstrual cycle. In reproductive medicine, a reliable test is needed to calculate the number of eggs a woman has remaining at a point in time as well as the quality of those eggs. The term "ovarian reserve" is used for this purpose. It is an estimate of the reserve of the woman's remaining egg supply. So, a woman's ovarian reserve determines her remaining fertility potential.
Antral follicles also referred to as small follicles (about 2-8 mm in diameter) can be seen, measured, and counted with ultrasound. Vaginal ultrasound is the best way to accurately assess and count these small follicles. The antral follicle count along with the age of the female is by far the best tool to estimate the ovarian reserve. A count of less than 4 antral follicles is considered an extremely low count with a normal of about 15 to 26. Fertile women who have an antral follicle count of 20 to 40, regardless of age, could reach menopause about ten years later. An otherwise fertile woman whose antral follicle count is only ten is likely to become infertile very soon. Women who have antral follicle counts of less than five are very unlikely to be able to get pregnant with or without infertility treatment.