Heija: Dear Mrs. Purdy,
Allow me to defend the "Mom"sayers among us. I, too, have noticed that I am renamed in the presence of my kids. It's even possible that I have also addressed other mothers in public as "Mom" or "Grandma" to avoid saying ma'am, or something truly insulting like that. I would much rather be called Mom than to wait the extra time it would take these service professionals to learn (and slaughter) my real name. In fact, it is probably saving us all money to allow this practice to continue. Do you call the cable guy "Mr. Cable Guy"?
If you really feel strongly about the practice, may I suggest a little subtle revenge? First, try responding to regular service providers in a similar manner. For example, instead of Jeff, call the photographer "Photographer." If that doesn't work, go ahead and sign the check with your real name: "Mom."
P.S. There is one person who is not allowed to call me "Mom," and that's my husband.
Have a healthy relationship with your husband after the baby is born