It is because of the lack of tznius in today’s era that the OP is mistaken. Skirts and dresses are SUPPOSED to hide the outline or ‘tzuras haguf’ of a woman. Unfortunately today’s skirts do just the opposite. They accentuate the woman’s figure, hence the OP’s conclusion that pants would be more tzniusdik.
What I personally don’t understand is the tayvah of a pregnant woman to wear tight fitting clothing. I mean, like, hello? (If somebody could explain that, greatly appreciated.)