When to tell my new boss...

I start a new job on April 6th (hooray!). I am 11 weeks pregnant, so will be about 12.5 weeks when I start. When should I tell my boss? I know he can't fire me bc I am pregnant, but I worry he will always have some animosity toward me because I kept this from him. I will  have the baby after about 6 months of being there. My husband thinks I should tell him immediately, which of course makes me so nervous. Anyone else been in this situation?