The follicle does not expel, what is expelled is the egg. When the follicle reaches about 20mm in diameter, it will rupture and expel the egg. The egg is still in the germ cell stage, so it is very, very small and cannot be seen by the naked eye. If the diameter of the follicle is found to be about 18mm by ultrasound, it is already a dominant follicle and may rupture and expel the egg at any time, so you can choose to have intercourse 24 hours after the dominant follicle is found and you will have a good chance of getting pregnant. If the follicle reaches 25mm in diameter and still has not ruptured and expelled the egg, clinically most of them may be caused by follicular luteinization and further examination is recommended.