Keeping a potential pregnancy a secret..?

Okay, so... My yearly pap this year, oddly enough is scheduled right at the end of my TWw.

For whatever reason, my mom has decided that she would like to come along for this appointment.......

I'm 25, married , don't live with her.. and not on her insurance.

My husband and I haven't told that we are TTC, because it's apparently a lot harder than we thought it was going to be. (8 months and nothing but BFN's).

Since I could possibly find out that I'm pregnant at this appointment, how do I keep my excitement under wraps (if the test is positive.)?

My husband and I agree on not announcing until I am about 13-15 weeks pregnant, if it ever happens.

I told my mom that she doesn't have to come, yet she insists.