Maternity sizes work the same as regular clothing sizes. That is, if you're normally a size "small," you'll still be a small in maternity sizes. Same thing with numbered sizes: If you were an 8 pre-pregnancy, look for items in that size.
And when it comes to jeans, particularly the designer sort that are labeled by waist size (for example, 28-inch), you'll still shop for a pair in that same size – just the maternity version. (True, your waist is no longer 28 inches but designers make it easier to find your current size by sticking with your pre-pregnancy number.)
Click here to see several charts for a quick guide to finding the size that fits you best!