3 year old step daughter keeps punching me in the face

Any ideas on how to get my 3(almost 4) yo SD to stop punching me in the face???? It’s 70% if the time a complete accident cuz she has no regard for others or what she’s doing and I know she’s three. But I’m so seriously sick and tired of being punched in the face by her I’m starting to get really pissed off when it happens cuz it happens multiple times every day. My SO says she’s three and to just get over it. Well, I’m pregnant with my second and I’m not just gonna sit around and wait for her to “accidentally” punch my stomach. Soooo idk what to do.