First, if you're worried about activities that could damage the baby, then you obviously already care about it's well-being so I don't think an abortion is the right option for you. But if you are going to have one, make sure it is for the right reasons- and hiding this from your Master is not the right reason. He will know anyway, the recovery from the procedure would give you away. You can't hide this from him; that would be wrong on all kinds of levels; BDSM, vanilla, and otherwise.

My advice is to tell him as soon as possible. Waiting can only make it worse. He's going to react how he's going to react; the way you present the news isn't going to make all that much of a difference; either way, he has to know.

No matter how this affects your BDSM relationship with him, you two are going to have a lot to figure out. Whether you decide to have an abortion, put it up for adoption, or raise it, that should be something that the two of you discuss together.