A babies new crib mattress should be just that... new. It should be firm and fit in the crib snuggly. Before you head out to the store to purchase your new crib mattress you should measure the crib so that you can make sure that you get a mattress that fits snuggly so that the infant can not get in between the mattress and crib. Remember always buy new. No matter how tempted you may be to pick up a mattress from a friend or snag that deal you found on the used mattress you shouldn't. This is one of those super important items that you want to splurge on!