Embryo freezing (cryopreservation) allows further attempts at pregnancy after an IVF treatment. The couple may have had a successful IVF pregnancy and wish another child some years later, or frozen embryos may be used a few months after an unsuccessful IVF treatment. Embryos are stored in liquid nitrogen at minus 198 degrees Celsius, and in Nigeria can be kept in storage for up to ten years. The science of cryopreservation has advanced in the last few years and pregnancy can occur in 15 to 20%of attempts. Not all embryos are suitable for freezing: poorer quality embryos will not survive the process and it may be better to repeat another 'fresh' IVF cycle.