First Trimester: Is great because most of your your regular wardrobe still fits. Unless it's your second, third, etc pregnancy and you might start showing sooner.
Key pieces: Bellyband as your work pants and skinny jeans will start to feel snugger.
Second Trimester: Congrats you've progressed from: "Is that a food baby?" to "Look at your cute bump!" It's officially time to showcase the belly, wearing huge tops will only make you look frumpy, work the bump!
Key pieces: Bellyband, Maternity Skinny Jeans, Leggings, long side ruched tops, long tank tops, and button down shirts (chambray).
Third Trimester: It's all about comfort at this stage.
Key Pieces: Dresses are great at this stage, maxi dresses & skirts, loose tops & tunics, and long cardigans.
Favorite maternity stores: Asos Maternity, H&M, GAP/Old Navy, & TopShop.
Other suggestions: Accessorize: Nothing distracts from a huge baby bump like a statement necklace, stacked bracelets, scarfs, and belts. And while they can't always hide the bump, they will make the mommy-to-be look more glamorous.
Layering: Try cardigans and blazers over a fitted top. It'll accentuate your smallest part and add some depth to your outfit.
Need more maternity outfit inspiration, here are some cute Maternity style blogs: MyChicBump; The Daybook; Tres Chic Mama; The Simple Swan