Consider an empire waist for all bridesmaids. This will make finding a dress for the pregnant bridesmaid much easier and will probably make her feel more comfortable.
Determine how far along the bridesmaid will be on the day of the wedding. If she is under four months along, she may simply be able to wear a bigger size of the same dress the other bridesmaids are wearing. For bridesmaids over five months pregnant, you will have to explore other options.
Choose the bridesmaids' dresses and ask if the style you choose comes in maternity. Sometimes you can get the same dress in a maternity version.
Find a maternity dress that is similar in style and color to the other bridesmaids' dresses if there isn't a maternity version of the dresses you've chosen.
Buy just the fabric as a last resort. If you can't find a maternity dress and you don't want your pregnant bridesmaid to wear a different dress, have her dress specially made.