Which sort of effects would there be on a 3rd trimester fetus from dental radiation?

Typically none: The dentist will place a protective apron over you to block exposure of radiation. If in doubt talk to your ob/gyn doctor and your dentist. Or if you are seeing the dentist for routine (non-emergency) appointment, and you still have concerns, possibly you can wait the few month for the x-ray).

Dr Fowler agreed

Little to No: Usually, it is recommended to delay routine, scheduled dental treatment in the 1st and 3rd trimesters. The 1st: because any radiation, medicines, drugs, etc can have the largest effect. The 3rd: because of increased discomfort in laying on your back, as it occludes the aorta and inferior vena cava, causing discomfort. If an emergency, then take care of it at any point, consulting with your dmd.

Dr Ali agreed

Probably none: But dentist will do x-rays during 3rd trimester only in emergency situations. Just make sure you are properly shielded during x-rays, and don't worry.

Dr Burns agreed