Best to avoid. It is recommended that pregnant women avoid all alcohol consumption, once they know they are pregnant and champagne has alcohol.
Just play it safe. Although there is some small amount of alcohol that may be safe during pregnancy, would you want to chance it? It's hard to quantify how much is safe & how much will harm a baby. At what point will the damage from alcohol be profound & at what point will it be mild? Fetal alcohol syndrome is serious & can be avoided by playing it safe. Don't take chances with your baby, don't drink alcohol.