White spots on teeth won't go away - what could it be?

It could be fluorosis, enamel demineralization, or early-stage tooth decay - either way, you need a dental exam because you can't figure out the exact cause on your own! Don't put it off, you should see a dentist as soon as possible.

Don't panic and assume the worst right away - white spots on teeth aren't always cavities. First off, they could be leftover plaque or mineral deposits that just don't come off well with a regular toothbrush. I'd try brushing that area more thoroughly first, maybe switch to an electric toothbrush or use whitening toothpaste.

But if nothing changes after a week or two, then yeah, go see a dentist. He'll figure out in five minutes what it is: fluorosis, early decay, or something like hypermineralization. It's worth waiting only if the spots aren't spreading and your teeth aren't getting darker.

Do the spots change color when you try to scrape them off with your fingernail?

If they really don't come off and have been there for a month, then it's most likely enamel demineralization or early-stage caries in the "white spot" phase - these are different things, but in both cases the tooth structure is compromised. I had something similar appear on one of my teeth a few years ago - it seemed like nothing serious, but when I went to the dentist, it turned out I needed remineralizing therapy, otherwise it would've developed into full-blown caries.

You definitely need to see a dentist, because you can't tell by eye what's actually going on. If you catch it in the early stage, you can handle it painlessly and cheaply.

If they haven't gone away after a month and don't come off when you brush - that's not just buildup anymore, you need to see a dentist. Only they can give you an accurate diagnosis, and you shouldn't put it off to avoid having to get treatment.