During pregnancy does the stomach or bikini line get hard first?

Both: I have seen the linea nigra which is the vertical line up the belly as well as old surgical scars /bikini lines darken during pregnancy.

Pregnancy: Everyone is different and there is no one answer here, but realize, as your baby starts to grow inside you, depending on your body build (skinny/overweight, etc) the lower part of your Abdomen where your bikini line is will often be the first part to start showing and getting "hard" as you say.