When to tell employer??

I teach yoga and fitness classes (13x a week!!) I found out I was pregnant 1 week ago and I've been out sick with a bad cold. I'm finally back and teaching my classes this week and I don't know how I'm going to do this!! The yoga is better because I don't have to model as much, but I can already feel that I have to modify my fitness classes cause I get so winded. I'll probably need to drop a few classws, but I have no idea which ones or when to do that. I guess I'll just take it week by week, and say something when I need to.

When will you tell your employer??