For the basics, I rely on GapMaternity/OldNavy Maternity.  They have really great basics that you can layer under shirts or blazers and give you some extra comfort/room to grow.  I highly recommend their PureBody line and their leggings.  I also really do like their maternity pants - the one area  that you really can't get away with wearing your old pieces because of the waist thing at some point.  When I was pregnant with Liam, I swore that I needed my designer denim.  Don't get me wrong, designer denim is great, but from my perspective, it's not worth the investment to wear for 6 months and then to pack away.  I have several of the long sleeve Pure Body Tees and Tanks in black, white, and grey and then several pairs of leggings and denim to rotate.  If you are pregnant during the cooler months, I love the Assets by Spanx maternity tights.

For add-ons, I shop at the regular places that you'd usually find me when I'm not pregnant.  I look for pieces that are generously cut (fit and flare is one that I find works very well) or have some stretch and/or ruching to them.  I generally buy at least a size up depending on the sizing of the piece and this also allows for some transition pieces as I get back to my new-normal body post-baby.  My other general rule of thumb is add a topper of some sort: a blazer, a vest, a jacket.  It helps provide some definition to your body.

Lastly, I had a good friend recommend to me that when in doubt, throw on a good statement piece of jewelry and/or a bright lip.

All of this is to say, I don't want to downplay how tough pregnancy at multiple different levels.  You are going through a ton of change both mentally as you prepare for your expanding family and physically as you are growing.  For someone who hasn't always had a great body image, I can fully appreciate how impactful the latter can be.  I think that's another reason why I try to stick as much to my 'normal' as possible when it comes to my clothing; it gives me a sense of stability in an immense time of change.  I'm sure that likely sounds a little silly, perhaps a bit vain and non-important in the grand scheme of things when you are growing a human being (or two).  Pregnancy truly is a beautiful (and sometimes very weird) process and should be celebrated; it's something that I don't take for granted.  It is something, however, that I don't think you should lose yourself in and everyone should find a balance between their comfort zone in their evolving self and their old self in every aspect.