^ same as PP, I would be worried that my OH might be hurt that I told someone before him, even with the aim of being a birthday surprise.
That being said, my sister knew before my OH both pregnancies, only because she was stalking my chart diligently and I told him face-to-face after work. He wasn't hurt by this, because he knew she stalked my chart rather than me telling her before him. But he did mention that he was glad I saved our big news for him first, then we called our parents together.
So that may be something you want to consider before telling your mom. It just may be something that hurts him, especially if this is your first.