(Someone slicing Maggie's stomach, killing the fetus, which becomes a zombie fetus, which then proceeds to eat Maggie from the inside out, Alien-style, falls under "whatever")
That's not how this works! That's not how any of this works!!
Seriously, though - a surface slice to the abdomen wouldn't impact the progress of a pregnancy. And a zombie fetus at this stage of development wouldn't have teeth. Or a jaw. It'd be the size of a grape.
I don't even think she got sliced anway - only her shirt. I think the shirt slice was supposed to unleash her motherly rage or something.