Follicle Size Follicle size is one of the important factors doctors monitor during IUI and IVF treatment. A follicle is a small fluid-filled sac in the ovary that contains a developing egg. As the follicle grows, its size can provide valuable information about egg maturation and help doctors decide the right time for ovulation triggering, IUI, or egg retrieval during IVF. However, follicle size is only one part of fertility assessment, and treatment decisions depend on severa