To confirm if an eggs are fertilized or not, this light will let you see then by showing a dark spot and veins. You just can't tell before then. You can mark your eggs with dates in pencil so that you can keep track. This Egg Candling Lamp was very helpful and was really fun to watch the growth of the eggs. You can check about once or twice a week to make sure that they were still growing. Features -Color:Silver -Material:Stainless Steel -Size:10.50X1.20X1.20cm/4.13X0.47X0.47in -Battery: AAA Battery(Not Included) -Professional and practical design with beautiful appearance, Egg Candling Lamp is easy to carry and use for a long time -The egg uses high-penetration and high-brightness lamp beads, that is convenient and practical, -After the incubator has been running for a few days, it's possible to scan the eggs with a bright light to see inside the shells. -When the lamp is , the hatching degree in the egg can be clearly seen, and the hatching degree of the egg can be easily judged. -Light egg candler is a product that checks whether the egg is fertilized and grows healthy, offer you a big help. Package list: 1 x egg lighting lamp