The first trimester I was able to get away with the flowy top/leggings combo and not even look pregnant. The little bun I had in the oven at that point was very concealable.
Second Trimester hasn't been that simple. I have a more obvious bump and if I go too flowy with the top it looks like a tent and my more form fitting tops no longer fit. I have found that leggings still work like a dream but for a top I need to do something fitted around the bust that flows around the tummy. Empire waists and "baby doll" tops/dresses are ideal.
My biggest advice when dressing during pregnancy is find what works for you and go with it. If leggings are rocking your world stock up! Be sure you are comfortable and don't let this be a time that you CAN'T dress nice and feel good about yourself. Treat yourself to something that makes you feel good.
Also realize that often each new stage of pregnancy is an experimental process. You may find things that worked one month no longer jive the next. Embrace the change and remember that it is not permanent. Cut yourself some slack and try to enjoy this time. Focus on colors you love and textures that are breathable. If you FEEL good you will LOOK good. :-)