Hi Moomin. I'm almost 15 weeks now and have been wearing maternity jeans for about 2 weeks, although they're still a bit too big and I can just about fit into my own jeans still! Although they do have to be unbuttoned after I've eaten or been sitting for a while!
I bought maternity trousers for work really early as my own trousers were really getting tight and i didn't see the point in buying a bigger size to tide me over for a few weeks.
Why not try them out and see how you get on with them?