I bought one pair of maternity tights and they were rubbish, kept falling down and were rather expensive to boot! Since then I've stuck to normal tights in a larger than normal size. I usually go for Boots own, either medium support or ladder resistant.
I'm 33 weeks now and currently wearing a normal pair with no issues. Well, apart from the huge ladder on the left shin...Not the ladder resistant ones!
I've just got some from mothercare at 34 weeks now my bump has exploded but I think before then they definately would have just bagged/fallen down and driven me mad. I could get away with normal ones in a size up until then so you might be OK?