Since almost all adults are immune ,there is no known risk if you are pregnant.Most people have antibodies by the time they are 4 years old .Good luck
This happened when I was pregnant with my second child. My older son got roseola (not fun and I feel for you taking care of your three year old during the couse of the virus---- the rash stage being very hard). I would absolutley tell your ob/gyn. The risk is minimal but if you are not immune, they have more of a concern. Problems that have been associated are in women much farther along in pregnancy.
But I too remember being worried. My doctor was reassuring and yours probably will be too----- so tell them at your next appt. or if that is not for another month, then give the office a call just for peace of mind.
Good luck and hope your three year old is all better. Happy pregnancy!