It's unrealistic to expect that "nothing will change" when he has a baby. Babies change everything! But it doesn't have to be for the worse. If you still love each other, and if the other gf isn't freaking out on you or anything, then just take it one step at a time.
It's not up to the bf's mom to tell you you'll have to "let go" (assuming that means "leave"). It's not her relationship, she doesn't even get a vote. If you three are happy, that's all that matters.
There's no reason you can't also have kids with him, if that what everyone wants. Lots of people do it. Three years is a long time, a big investment, and if this is the only reason you feel you have to leave, then don't make any rash decisions. Just take it a day at a time, see how things work out, and as always keep talking it out!
Best of luck. Whatever happens, life will work itself out. It always does, if you allow it.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker