In order for an egg to ovulate during a menstrual cycle, it has to first developed within a small cyst within an ovary. As this egg starts to mature, the cyst that surrounds it will also grow bigger; we now call this cyst a follicle.
When a follicle reaches a diameter of 18 to 20mm, we assume the egg inside is mature and ready to be released via ovulation.
When a follicle fails to grow to at least 18mm, and remains smaller than that during an entire menstrual cycle, the egg inside is deemed immature, and the follicle is called an immature follicle.
Immature follicles (and therefore immature eggs) are usually of poorer quality , and may not be fertilised easily. Hence this leads to some reduction in the fertility potential of the lady. However in many ladies, such situations only occur occasionally; there may still be normal cycles with normal mature follicles/eggs which can ovulation and be fertilised normally.
In those with recurrent monthly immature follicles (eg polycystic ovarian syndrome) medication may be required to help these ladies overcome their fertility problem.