The rubella virus causes a mild illness but can seriously damage an unborn baby.

http://www.patient.co.uk/health/planning-to-become-pregnant

The rubella virus causes a mild illness but can seriously damage an unborn baby, especially in the early stages of growth. So, until you know that you are immune (from the result of the blood test), you should avoid anyone who has rubella, especially in the first 16 weeks of pregnancy.