Hi dear,
IgG is tested to detect your immunity to a particular infection ie, your capacity to fight that infection , negative means you can not fight it right now and if you catch this infection by any source , you will have the disease and if
pregnant can transfer it to the baby as well which can be quite serious at times
So , get immunised ie, get the Rubella vaccine but first make sure you are not pregnant before it ( best way is get vaccinated during menses ) and avoid pregnancy till 2 months after vaccination.
Thanks,
Dr Astha Jain Mathur