Firstly, congratulations. Hope your betas double, and everything goes well for you.
The two dvds that I used to do were:
Shiva Rea prenatal yoga;
Erin O'Brian prenatal fitness fix.
wrt core work - it only really matters once you are in the second tri. It is important not to lay flat on your back for long periods.
Hope you enjoy these. I am partial to shiva rea, as I really love her yoga practices.