The MMR vaccine is a live attenuated virus vaccine. In a severely immunocompromised person (including unborn children), it may cause disease. Disease then carries the potential complication of encephalitis. We don't give live vaccines to pregnant women or those who are severely immunocompromised. The conditions under which the vaccine could/might cause it are extremely rare (this is not an assumption - see
http://www.cdc.gov/vaccines/vac-gen/...ffects.htm#mmr), but every possible risk of adverse reaction must be listed on the insert. Encephalitis is not a material risk of the MMR vaccine. We, and our children, face much higher risks from the disease.
One of my children developed varicella (chicken pox) encephalitis when he contracted it at 8 years old. There was no vaccine then. It was a horrible thing to watch him go through.