What to look for when buying
The right size: A mattress needs to fit snugly in the crib, with no space between the side of the mattress and the crib frame. If there's a space, the mattress is too small and could be a suffocation and entrapment hazard. The size of both crib mattresses and cribs is standardized by the federal government, but due to slight variations in each, not every mattress will fit perfectly in every crib.
Firmness: The firmer the crib mattress the better (mattresses designed for older children and adults may not be firm enough). Even if it feels very stiff to you, your baby will adjust to it. Consumer Reports suggests this test: "Press on the mattress in the center and at the edges. It should snap back readily and should not conform to the shape of your hand."
Density: You want high density so it’s firm enough to keep your baby safe while sleeping. Most foam mattresses don't list density on the packaging, but weight can be a good indicator. As for innerspring mattresses, manufacturers often equate the number of coils with firmness, but the gauge of the wire is just as important. Lower gauge means thicker wire, which is stronger and therefore firmer. Look for a mattress with 135 or more coils and a gauge of 15.5 or lower.
Resiliency: When you push your hand down into the middle of the mattress and remove it, how quickly does it regain its shape? Faster is better; sleeping babies make an impression on the foam, and it can be difficult for them to change position if the mattress retains their shape. Some foam mattresses are “2-stage” or “dual firmness,” with a firm side for infants and a softer side for toddlers.
Weight: A typical foam mattress weighs about 7 to 8 pounds, although mattresses made of memory foam (an especially dense form of polyurethane) can weigh close to 20 pounds. Innerspring crib mattresses are heavier in general, weighing in at about 15 to 25 pounds. Keep in mind that you'll be hoisting up a side of the mattress, or lifting the whole thing, when changing your baby's sheet.
Mattress cover (ticking): For water resistance, look for double- or triple-laminated ticking reinforced with nylon. This composition is also more resistant to tears, holes, and soggy diapers. Organic mattresses usually have cotton covers; parents may want to consider a fitted waterproof mattress cover.
Venting: Look for small holes on the sides of the mattress that let air flow in and out. A mattress will smell better if it has plenty of vent holes to let odors escape. Diapers do leak, so this is important.
Cleaning: Most traditional mattresses suggest spot-cleaning only. Some have removable covers that can be machine-washed. The innards of at least one crib mattress on the market can be hosed down in the tub once its washable cover is removed.