An embryo can be frozen during several stages of its development: from the day of fertilization, when they are an only cell1, to five or six days later in their blastocyst stage 2, when they are the same size but with more cells within. During their first seven days alive, they grow within the external membrane of the oocyte, exactly like a chick in an eggshell.
Remember an egg is a chicken’s oocyte! The moment in which the cells break the membrane is known as the hatching of the blastocyst 3, they come out and implant immediately in the inner layer of the uterus. Thus, they can befrozen up until that moment.
Most of the time, a frozen embryo transfer takes place when there are “extra” embryos after a conventional IVF cycle. A “fresh” transfer is usually preferred. However, some doctors are recommending elective frozen embryo transfer—also referred to as a “freeze all” approach—where afresh transfer is not attempted. In this case, all embryos are cryopreserved and transferred in a FET cycle in the next month or so.