Michelle Emblenton, biochemist at inviTRA, tells us about the antral follicles:
Antral follicles are the ovarian follicles that still contain an immature egg inside them. However, these follicles are developmental stage that they are sensitive to follicle stimulating hormone. As such, these anthropolicles may develop into pre-ovulatory follicles. From these antral follicles only one will develop each month to then release the mature egg in ovulation. Antral follicles are characterized by having something in them called antrum and this is a fluid-filled cavity within the antral follicle. This allows antral follicles to be visualized in an ultrasound examination and indeed they can be of one to two millimeters in diameter. So using an ultrasound the specialist can visualize the antral follicles and actually count them.