M.R.
I have an adult heart murmur.
Yours has probably been there for quite a while, this is just the first doc that picked up on it.
Unless you have had a recent serious illness with fever? I had Scarlet Fever in my early 20's, which caused the scarring and ultimate murmur.
It has not changed my life one bit. There are different levels, and once yours is higher than a level I, you and your dentist should discuss prophylactic antibiotics.
Don't worry about anything yet. Just have your OB follow it and mention it to other doctors you visit, like family, internist.