Oops!

So we're 10 weeks and 5 days pregnant! Yay! However, we aren't telling anyone (besides about 5 necessary people) until May 21st. That week we're having our 3D done and finding out the gender. So we invited everyone over for our annual summer kick off BBQ and have decided to turn it into our Baby-Q! We're telling everyone we're pregnant and doing our gender reveal all in one. 
Problem is I didn't think of the fact that I'll be about 15 weeks by then and I'm probably going to start showing soon. Any recommendations on how to hide the bump but not be conspicuous about it?