At 3 months your baby might be teething -- most babies start teething between 4 and 7 months. But at this age, a more likely possibility is that your baby has started to "find" her hands, which may become her new favorite playthings. Once that happens, putting them in her mouth is part of exploring them and discovering that they are part of her and under her control. It's a great sign of growing up. Just one word of warning: Once your baby's mastered the hand-sucking thing, that means she'll soon be skilled enough to hold other things and put them in her mouth. You'll need to start being more vigilant about babyproofing and always keeping an eye on her.
Copyright 2002 Meredith Corporation. Updated 2009.