According to the manufacturer:
"The lowest possible therapeutic dose is recommended if treatment with Lamilept during pregnancy is required."

"Physiological changes during pregnancy may affect the concentration and therapeutic efficacy of lamotrigine. There have been reports of a decrease in lamotrigine plasma concentrations during pregnancy, with a potential risk of loss of seizure control. Lamotrigine concentrations at birth may increase rapidly, with a risk of dose-dependent side effects. Therefore, serum lamotrigine levels should be monitored before, during and after pregnancy, as well as immediately after birth. If necessary, the dose should be adjusted to maintain serum lamotrigine levels at the same level as before pregnancy or adjusted according to clinical response. In addition, dose-dependent side effects should be monitored after birth."

The dose is determined by a neurologist, usually it is not the responsibility of the gynecologist.

More here:

https://www.gdziepolek.pl/artykuly/jakie-leki-mozna-stosowac-w-ciazy