Dentist told me to use fluoride rinse when pregnant. Used it for a few weeks in first tri. Harmful to baby?

Keep using. Fluoride is good idea and will not cross the placental barrier, so baby ok. Because of hormone swings during pregnancy if gums get irritated they get swollen, making it harder to clean. Make sure you brush 2 minutes (by the clock) 2x/day and floss at least 1x/day, every day. See your dentist for ck-up & cleaning during your 2nd trimester. Have ck-up & immediate after birth. Congrats!
Don't swallow. If you are worried, make sure you don't swallow and there is now way it can hurt your baby, but it can still help your teeth.
Fl rinse. Using a Fluoride rinse during pregnancy will have no harmful effect on your baby. The rinse will reduce the risk of cavities and help prevent gingivitis which is common during pregnancy.