Hi Jill-
Congratulations on the pregnancy! You have twins? Twins are great (I'm a twin too!). When I found out I was pregnant with my first baby, I was on mysoline and my doctor took me off that in a hurry. It's a class D and I had at least 3 ultrasounds to make sure the baby didn't have a birth defect from the drug. Luckily, the baby was healthy.
Since my menses seemed to trigger my seizures in the first place, my ob and I decided not to substitute another antiseizure drug for the rest of the pregnancy and while I nursed the baby. The thought of starting a new drug while going through my first pregnany was traumatic. If the seizures returned, I would just resume another medication. Again, Luckily, everything went fine. I had a close working relationship with my doctor and it made all the difference in the world. My pediatrician was super too.
For me, if I was on a class C at the time I found out I was pregnant, I think I definitely would have stayed on it. It would have been less complicated and without the worries I went thru. Bottom line... I was glad my doctors were there for me every step of the way). Hope you have an uncomplicated pregnancy!
Take good care of yourself, Hugs, Glenda 
complex partial seizures since 22