I'd probably hold off until after your pregnancy or at least until your second trimester. I've got a tooth whitening kit and did my teeth a few months before getting pregga's. You're advised to do a touch up every six or so weeks, but I'm just letting mine go until after bubs comes.

The active ingredient in the whitening gel is actually peroxide. The reason they advise you not to use is when preg or bf'ing is because there has been little or no research done into the effect this has on bubs. It's not just ingesting it that you have to worry about. The solution is up against your gums which means it gets absorbed it into your blood stream. The amount isn't enough to hurt you, but they don't know if it will effect bubs.