Human eggs or oocytes grow in the ovary in structures called follicles. During an IVF cycle the growth of follicles generally tells the physician whether or not the eggs are becoming mature. But the doctor cannot see an egg in the follicle because it is too small to be seen using ultrasound. Usually however, mature eggs come from follicles bigger than 14-16 mm in diameter. When eggs are removed from follicles on Day 0 (the day of egg retrieval) it is difficult to tell if they are mature or not because they are surrounded by a mass of cells called the cumulus. If we remove these cells and look very closely at the egg we can tell how mature it is. A mature egg has a small structure called a polar body that lies next to the egg, beneath the outer shell, or membrane, that surrounds the egg. An intermediate stage egg will not have this polar body. An immature egg has a structure inside it called a germinal vesicle.