A crib mattress is used in the bottom of an infant's crib to provide comfort and safety. The firmness and size of the mattress are the most important factors when choosing the perfect mattress for an infant. Mattresses should be as firm as possible to help prevent Sudden Infant Death Syndrome or SIDS. Before purchasing a mattress, it should be tried inside of the crib it will be placed in. After a mattress is installed, if you can fit more than two fingers on any side of it, the mattress is too small. It is important that the outside of the mattress is water-proof to assist with clean-up of spit-up or leaking diapers.