It's important that you buy a mattress for your baby that's firm. If a mattress is too soft, it increases the risk of SIDS. This is because your baby can sink into the soft mattress texture and suffocate. You can test to see if a mattress is firm enough by pressing around the entirety of the mattress to see how much it stinks in. If it sinks in too much, your baby won't be able to move around or turn themselves to breathe.