The potential risk could be to the baby's ears as they can hear quite well from in there even the sounds of a normal conversation. I'm not sure at which week the tympanic membanes develop so you might want to consult a OBGYN or pediatrician on this matter.
My wife and I specifically asked this question when she was pregnant with our first child. He told is that she would be just fine to shoot anytime she wanted. He did say she may want to stay away from anything with high recoil for the obvious reason of not repeatably jarring her body and no trick shooting off of her belly! Lol. As everyone said, ask her doctor but that's what her doctor told us. He also said of we asked a doctor who was anti-gun they would probably tell us exactly the opposite but there was medical reason not to.
I was curious. I had never heard that from "back when" our OB/GYN did say no heavy recoiling shotguns/rifles but handguns were fine until late into pregnancy then there was some risk of hearing damage.
I sure don't blame you for not risking it, I'm sure we'd not either had we been told that.
I don't see it hurting the baby at all. Maybe a .22 cal would be better if she must get it now. but it is your kid she is carrying. Is it worth risking it at all? Just wait till after it's born. Kids are priceless.