What Is a Follicle?

A follicle is a small, fluid-filled sac in the ovaries that contains an egg cell. In each menstrual cycle several follicles begin to develop; in a natural cycle usually only one matures and releases its egg at ovulation.

In IVF treatment the aim is to help several follicles mature using hormone medication (ovulation induction). The number and diameter of the follicles are monitored regularly with ultrasound throughout treatment; once they reach sufficient maturity, egg collection (OPU) is planned.

The small follicles counted by ultrasound at the start of menstruation (the antral follicle count), together with AMH, are among the most important indicators of ovarian reserve and guide the choice of treatment protocol.

Key Points

• A sac that holds the egg cell
• Number and diameter tracked by ultrasound
• Antral follicle count is a reserve indicator
• Once mature, eggs are collected via OPU