First of all it is generally recommended that Dental X-Rays, Particularly in Pregnent Ladies during first and second trimessters (1-3 and 6-9 months), must be AVOIDED untill and Unless deemed absolutely necessary or being in sort of medical/ dental emergency.
exposure from a dental xray after absolute Shielded protection is very minimal and is equivalent to normal Background radiation from earth, heated by the Sun Rays.
In your case as, you are near full term, the most of the baby's intrauterine growth is near completion, and as your dentist
had provided you with extra shielding, before taking Xrays, you should not worry about any sort of detrimental effects on your baby.
there is relatively very very rare exposure to baby after such protection. Don't WORRY!!!!!
and Congratulations and best wishes for a healthy baby....