Can antibiotics harm my unborn child?

Maybe. There is evidence that some antibiotics may have fetal interactions. Best check with your doctor before taking any.
Varies. There are some antibiotics that could interfere with bone formation and stain babies teeth. There are some that would make baby more prone to jaundice if given close to the time of delivery. However, if your doc knows you are pregnant, they select meds that pose no risk for baby.
Generally. No, however please check with your OB as some infections may require a different regimen.