They are the small sacs filled with fluid that are found inside a woman’s ovaries.
They generally secrete hormones that influence stages of the menstrual cycle and women begin puberty with about 300,000 to 400,000 of them.
Each one also has the potential to release an egg for fertilisation.
Follicles and their size and status are a vital part of assessing fertility and fertility treatment. In a normal menstrual cycle one follicle will grow containing one egg.
The follicle will also get bigger until it ruptures at ovulation which then releases the egg.
This takes usually around 14 days after the beginning of the menstrual cycle.