What oral antifungal to use against trichophyton rubrum when you intend to get pregnant soon?

None: None of the oral anti-fungal medications have been studied in the first trimester of pregnancy and determined to be safe. They are rated category B: animal studies have shown no adverse fetal effects, but controlled human studies haven't been done. Fetal harm possible but unlikely. Since fetal harm is possible, and nail fungus treatment isn't urgent, would not use it in the first trimester.

Terbinafine: Terbinafine has been found to be effective for treating fungal nail infections, is listed as Pregnancy Category B ("Animal reproduction studies have failed to demonstrate a risk to the fetus and there are no adequate and well-controlled studies in pregnant women") & considered to be reasonably safe. Do discuss with your obstetrician before starting.