The general advice is a 3 month gap between getting an MMR shot and starting to TTC. Although I have read some countries where its only 1-2 months but in Aus they say 3. The docs can get your rubella immunity measured with a blood test (rubella titre).

Mine was "equivocal" which mean somewhere between immune and not immune, so I had the MMR shot 3 months before TTC. Then I was retested 5 months later when I fell pregnant - and I was still equivocal... They will give me another one after this baby arrives but it might be normal for me to just always turn up equivocal no matter how many shots I have... everybody is different.

As another poster said, the first 12 weeks is the real danger period. I think from about 16 weeks the chances of an infection causing any problems are low because most organs have developed by then.