You may find this page helpful which has a section on pregnancy:

 http://www.rxlist.com/cgi/generic/carbam_wcp.htm

From my reading of it you're stuck in a bit of a cleft stick: if you stay
on carbamazapine (=Tegretol) and especially in combination with other AEDs
as you take it, not monotherapy, then there's a suggestion that your child
has a higher risk of congenital malformations, including spina bifida. Their
conclusion is that "Carbamazepine should be used during pregnancy only if
the potential benefit justifies the potential risk to the fetus."

That's going to depend on how much control you get without Tegretol and what
sort of seizures you get, if any. It all seems to a bit empirical but it
looks to me as if your unborn child could be in as much, if not more, danger
if you have full tonic/clonic seizures while you're carrying it.

Oh yes, and as that page also makes clear, if you stay on Tegretol then
assuming you give birth to a healthy child the "fun" isn't over yet as, if
you breast feed there's a significant danger of the child having an adverse
reaction to it.

I think if it was me I'd come off the Tegretol. It sounds to me like the
main risk to the child then is it dying before it's born during a seizure.
That would be a tragedy but you could have another child. In the other
scenario if the child was born with spina bifida you would spend the rest of
your life going "if only I'd come off Tegretol like the doctor told me to".

Rock meet hard place, hard place this is rock :-(

Also in UK.

My wife has been on 800mg Tegretol Retard for 18 years.
Specialist would not move her from Tegretol until after all children were
born, claiming 'newer' AEDs are less well tested.  We were also advised that
having a fit was more harmful than the Tegretol.
Sue found she needed lower dose of Tegretol during pregnancy, and also
couldn't hold tablets down some days.
We have had no problems with development of any of our 3 children, despite a
couple of Grand Mals during early pregnancies and an ectopic one (ouch!).

We now find (via the internet) that Sue was supposed to avoid grapefruit in
conjunction with the Tegretol.  No mention of this from official sources!

Best regards

Chris