An egg is the largest cell in the human body. Sperms and embryos are technically easy to freeze. Egg contains a lot of water inside, and when frozen can form crystal ice, which can cause trauma to the cell. The newer techniques now, dehydrate the cell, and then frozen, thereby preventing crystal ice formation. This technique is called Vitrification used for cryopreservation of eggs or oocytes (technical word). Zoi Fertility only uses vitrification for all their egg freezing cycles.