When in vitro fertilization, or IVF, is used, the male's sperm is incubated with the female's eggs in the laboratory. The development of the eggs is medically regulated with hormones, and the conditions in the laboratory dish are meticulously controlled to optimize chances for fertilization. The eggs that are fertilized (i.e., "embryos") can then be assessed on the basis of their appearance, allowing for the selection and replacement into the female of the best embryos. These embryos have been shown to be ones with the highest likelihood of resulting in a successful pregnancy. Embryos that are not used can be frozen (cryopreserved) for later use.