Frozen embryo storage. Technician removing frozen embryos from storage for in vitro fertilisation (IVF). They are frozen in liquid nitrogen at -196 degrees Celsius. For IVF, fertility drugs are given to potential mothers to stimulate the production of several ova (eggs), rather than just one egg as normal. These eggs are fertilised in the laboratory using a sample of sperm. If fertilisation is successful, 3-4 embryos are introduced into the mother's uterus to increase the probability that one will implant. Any surplus embryos can be frozen and stored for possible future use, as has been done here.