Is it bad for my baby's teeth if I let her fall asleep with a bottle in her mouth?

Yes. When your baby falls asleep while sucking on a bottle, the liquid will pool around her teeth as she sleeps, which can cause cavities.

This happens because the bacteria in her mouth change the sugars in milk, juice, formula, and breast milk into acids that can dissolve her tooth enamel. (This is most often a problem for the front four upper teeth because your baby holds the nipple against them with her tongue.)

If you've already gotten your baby into this habit and she won't go to sleep without a bottle, give it to her as you rock her to sleep and then put her down in her crib without it. If she's older than 6 months, you can dilute the bottle contents with water, so that it's less concentrated with the sugar that causes decay. If she balks, start by replacing only one ounce with water, then increase the amount of water by one ounce a day until you're giving her only water.

Your baby can also end up with tooth decay if you're breastfeeding her "at will" (especially if she sleeps with you and can nurse at will through the night). If she's feeding that often, her teeth will be constantly exposed to the sugar in the breast milk. I would suggest wiping or brushing the teeth and gums often during the day and before night if you continue to breastfeed this way.