The Monster star became a first-time mum in March (12) when she welcomed four-month-old boy Jackson.
But instead of readying her baby's nursery like most mothers, Theron was busy completing his U.S. passport application forms, which take several weeks to process, so the child could travel with her around the globe.
During an appearance on U.S. talk show Jimmy Kimmel Live! on Thursday (24May12), she said, ""Going through the adoption process, I knew when I was gonna have him come into my life so before I even had him, I had to start figuring out how to get a passport for him because I knew this tour was going to happen for the film.""
The blonde beauty kicked off her promotional duties at the film's world premiere in London on 14 May (12) and since then she's been to Spain, France, Germany, and Japan - all with her new baby by her side.
However, Jackson has already doubled in size since his arrival and Theron fears she may face tough questioning from Customs and Border Protection officials as his features begin to change from those shown in his passport photo.
She adds, ""We came through customs in L.A. and the agent held his passport next to his face and looked at me like I was smuggling another child in. This passport is valid for five years so I can't imagine (what it's going to be like in a few years)... I mean, babies change every day.""