Thanks for the query.
Ig M positive with Ig G negative usually indicates primary infection.
But as you have taken the vaccine
last year it is possible that there may be some error in the lab report.
So you wait for your next report and if needed go for Ig G avidity testing.
After getting the report we can come to one conclusion.
As the baby is good at present, no need to worry.
For details about fetal risk:http://srsree.blogspot.in/2013/02/torch-group-of-infections-rubella
For more details you can ask me through: http://www.healthcaremagic.com/doctors/dr-sree-gouri-sr/63429