As you are all well aware I am a fan of TOPSHOP and highly rate TOPSHOP's denim, so it was the first place I went for my maternity jeans.
I believe that it is worth investing in maternity jeans early on in your pregnancy - they are so much more comfortable on your expanding waist, even with a small bump so why wait? Also compared to normal jeans they will have limited wear (although I have friends who still wear theirs for a Sunday roast). You will most probably be wearing them for a while once the baby has arrived too - so get some you love.
Another thing to know is that maternity jeans can either fit under or over your bump. It is personal preference as to which you prefer but some style legs work better than others.
The advantages of over the bump is that it keeps your belly warm and you don't suffer from 'mum bum' when you squat down. Also you can wear shorter tops without exposing your belly. If you are looking to get a looser fit leg or a non-stretch denim jean then I would opt for over the bump as they will tend to stay up better - no belts remember!
The advantages of under the bump are that they aren't so bulky under tops and to combat exposure or to allow you to wear a shorter top you can always wear a belly band or long vest top under your top top. Under the bump tend to work better for skinny, stretchy style jeans as the stretch in the denim will hold the jeans on your leg as supposed to having to rely on the waist band only.
Up until very recently there has been a distinct lack of interesting style maternity jeans - and most still tend to be skinny which is fine and often what you feel like wearing as they tend to balance out the ever expanding top half of your body, but still it is good to have a choice of styles.
Here are my top 4 and what you need to know: (FYI I am about 6 months pregnant in these photos)
These are very comfortable and I love the cropped length. They are normal non-stretch denim but i've found them to be good at retaining their shape. Because they are a loser fit it is an advantage for them to go over the bump as this stops them falling down.