If you read any article with fashion tips, they always say one of the biggest fashion no-nos is wearing clothing that doesn't fit you properly.
There is no time when fit is more important than during pregnancy. Wearing "normal" clothes a few sizes too big can make you look a few sizes bigger, and frumpy. They will not fit you across the shoulders, the arm length will be too long, and if you are wearing pants, they will sag around your bottom - not very flattering!
By the same token, trying to fit into your pre-pregnancy clothes will not only be incredibly uncomfortable but can also look very tacky. T-shirts riding up, pants stretched to the limit with the top button undone... You know the look and it isn't good.
Just like you invest in new clothes each season, look to invest in maternity clothes for your "pregnancy" season. Plan your wardrobe and aim to get a few key mix and match maternity pieces. The investment will be worthwhile because you will feel so much more comfortable, and confident in your appearance.