Obviously they won't be able to wear the same shape of dress, but different styles in the same colour would go well. They could be accessorized the same, e.g. stole/shrug/head-dress/shoes etc. to bring the look together. I don't think it's necessary for adult bridesmaids to wear identical dresses anyway, pregnant or not. They don't have to be long either, even if the brides gown is long. You could look at cocktail type dresses.
If you can't find anything for the pregnant lady, could you find a dressmaker who can make something/modify a readymade maternity dress? You never know how big someones bump and boobs are going to grow, and it might be kinder to match the other bridesmaids dress to one that is comfortable for her.