One of the most essential maternity staples is the basic black legging. They're perfect for layering under dresses (as they are comfier than tights) and longer tunics that cover the bump. I highly suggest purchasing an actual maternity legging rather than any-old legging, because they are cut to accommodate the growing belly. As with most maternity pants, they come in two varieties: those that are lower cut that have elastic under the bump, and those with a large panel that pulls up over the bump, right under the chest.
A good friend gave me these amazing leggings from Hatch, an ingenious line of clothing designed to fit women through pre-pregnancy, pregnancy, and post-pregnancy. I absolutely loved these and wore them at least twice a week (thanks Jeanyoung!) from when I was in my first trimester through my third trimester. I loved how opaque they were, and the lycra material was nice and thick. I will admit though, that once hitting the late third trimester, they were a litttle tight on the waistband creating more pelvic pressure than I really wanted. Another great option is this legging from Ingrid & Isabel (maker of my other favorite maternity item, the BellaBand). I tried these on in the store, and they were suuuuper comfy. They pulled up easily over my third trimester belly too!
Outfit 1: Trenchoat: Zara (old). Gray striped maternity dress: Old Navy (tank version here, black version seen in this post). Leggings: 'The Premium Legging' by Hatch. Suede booties: AllSaints 'Hessian' Boots.
Outfit 2: Tunic: Rosie Pope 'Alison Blouse' in Wishbone print (spring version here). Leggings: Hatch. Necklace: Vintage optic lens. Sneaker wedges: Ash 'Bowie'.