My sister recently asked me to help her find clothes for an upcoming maternity shoot that she had. When I picked out clothing that wasn't from maternity stores, she was quite skeptical of my decisions.
Personally, other than the necessities (like maternity pants and bras) I don't see why a woman can't pick pieces that she can wear while pregnant and also use after she gives birth.
The pieces that I picked for my sister were all versatile and easily matched with other clothes that she already had in her closet.
I wanted my sister to realize that being pregnant didn't mean that she had to sacrifice her style. Nor did she need to feel like she had limited options of what she could wear.
Just to further prove my point, I did a photoshoot in the exact same outfit. Same size and all. Needless to say, my sister was happy with my decision to shop at non-maternity stores for her and she now is able to expand her wardrobe with pieces that she will use for more than just 9 months.