Q. Sometimes after my son finishes his midnight bottle, I fall asleep without putting the remaining formula away. How long can formula stay out without spoiling?
A. Opened formula, either ready-to-feed or liquid concentrate, shouldn't be left at room temperature longer than two hours—and no more than an hour if the room is warm. Ideally, you should get into the habit of filling the bottle near the refrigerator and putting the remaining formula back before the feeding—but, of course, there's a big gap between the ideal and reality in the middle of the night. Even properly refrigerated liquid formula should be used within 48 hours; if you've made a batch from powdered formula, you should use it within 24 hours. Remaining powdered formula should be used within a month after opening.
Any formula left in the bottle an hour after your baby has started feeding should be discarded, so try not to fill the bottle with more than he usually takes. And as with any food, particularly one meant for a baby: When in doubt, throw it out.
Contributing editor Heidi Murkoff is the coauthor of the bestselling What to Expect When You're Expecting (Workman) and the author of the What to Expect Kids series of children's books (HarperCollins).