During a typical IVF or ICSI procedure, the eggs are collected from the female partner. It is then fertilised and undergoes embryo culture. The best embryos so produced are selected for embryo transfer. Usually there will be good embryos which are in surplus to the ones required for embryo transfer. These embryos are frozen and stored for future use.
Embryo freezing is a method of preserving the embryos by carefully cooling them to very low temperatures (- 196o C). This is done using specialised freezing equipment and embryos are stored in liquid nitrogen for extended periods.