If you are pregnant, congratulations! I hope all goes well for you, and you enjoy this wonderful time. Remember that when shopping for maternity clothes, your first priority should be comfort. Mothercare has a good guide to buying maternity clothing here.
Next, Mothercare and Mamas and Papas all have really good ranges of maternity clothes. You can also find some maternity pieces at stores such as Topshop, Dorothy Perkins and New Look. You don't necessarily have to buy a whole new wardrobe though! If you buy a few maternity tunic tops and dresses, you can still wear pre-pregnancy cardigans and jackets over them, with the buttons left undone.
If you usually wear an abaya, consider having a few custom made. If you a know a really clever seamstress, she may even be able to take them in again after the pregnancy, so you can continue to wear them. Here are a few great maternity pieces: