The best crib mattress has firm support and fits snugly into the crib so there are no gaps whatsoever. These are the two most important safety features.
Firmness is necessary so your baby doesn’t sink into the mattress far enough to hinder breathing. Suffocation and Sudden Infant Death Syndrome are real dangers that can result from a too-soft mattress, especially for young infants who cannot turn themselves around.
A snug fit makes sure that your baby can’t get trapped or caught between the crib and the mattress. Standard Crib Mattress Size is set by law, and cannot be less than 27 1/4 inches by 51 5/8. You can easily test the fit by pushing your fingers between the mattress and the crib. If you can fit two fingers inside, the gap is too big. The thickness of the mattress should be no more than six inches.