Q Which is better -- liquid or powder?

A. Nutritionally speaking, powdered, liquid concentrate, and ready-to-feed formulas are all the same. Where they differ is in cost and convenience:

• Powdered formula is the least expensive and lasts the longest after opening (30 days without refrigeration).

• Liquid concentrate needs to be mixed with water, refrigerated, and used up within 48 hours once opened. It can be handy if your baby's exclusively or mostly formula-fed, but the cans are heavy to carry.

• Ready-to-feed is, well, ready for feeding: You open the can and pour out the formula. (You can store the rest up to 48 hours.) It's the priciest of the three and the heaviest and largest to store. But it saves you the effort of measuring and mixing.

Choosing the right formula depends on your needs, and your baby's. Some parents tell me that their babies handle one type better than another  -- they get constipated with the powdered, say, or spit up the concentrate  -- but these reactions are very individual. If you think your baby is having trouble with his formula, his pediatrician can see whether another type makes sense.