Follicle vs. egg (they are not the same)! Follicles are fluid-filled sacs in the ovaries. Eggs sit inside of these follicles. As eggs mature, their follicles grow in size. Each menstrual cycle, multiple eggs begin to mature, but only one (sometimes 2 in the case of twins) ends up maturing while the others degenerate. This egg is ovulated from its follicle (called the dominant follicle) under the influence of reproductive hormones. IVF medications alter reproductive hormone levels in order to cau