With each menstrual cycle, a set of ovarian follicles starts developing and growing, but only one of them will make it to the last stage of the process: ovulation. As follicles grow, they go through various stages as one shall see in this picture. Some of them will degenerate throughout this journey.
The main phases of follicle development include: primordial, primary, secondary, antral and preovulatory follicles. The one that reaches the preovulatory stage is the one that will mature and make it to the ovulation day.
Antral or Graaf follicles are marked by the presence of a fluid-filled cavity that allows specialists to visualize and count them by ultrasound scan.