Yes congrats on the pregnancy. I think it is a combination of alot of the vitamins, mainly the b complex vitamins that come out in your urine as a very bright yellow since the b vitamins are water soluable.
What ever the case, do not stop taking your vitamins even after your delivery. Even if you do not nurse the baby, it is a real drain on the mommy's system.
Enjoy preganancy, it is tough on us guys but it is even harder on the moms ;>)
brian