When the follicle is 18mm in diameter, it is considered mature and clinically known as the dominant follicle, which can rupture at any time and expel the egg. When the follicle reaches 25mm in diameter, if it still does not rupture and ovulate, it is clinically likely that it will not rupture and ovulate again. This condition is called follicular luteinization and is often caused by inadequate hormone production or thick follicular walls, or by an ectopic endometriotic lesion that is located on the ovary. Clinically, treatment is given either by increasing endocrine medication or by directly puncturing the follicle and expelling the egg under ultrasound instructions.