Can mono cause birth defects?

No. From cdc: "there are no known associations between active ebv infection and problems during pregnancy, such as miscarriages or birth defects.".
Not likely. Ebv rarely crosses the placenta and even more rarely causes birth defects to the heart/brain or liver/kidneys. Reports of such cases are very few and far between in the medical literature, suggesting true rarity and remote clinical significance (theoretical rather than real danger).