She was probably recorded as carrying an infant in arms when she came back to the US. IIRC, babies and children under a certain age didn’t have to have passports but traveled on their parents’ passports; they also were often not listed on passenger lists, either. Now, of course, everything has gotten a lot stricter, but at that time, I don’t think it would have been impossible for her to bring her baby back into the US under her passport.
I know that to be a fact. I got my first US passport in 1963, at the age of ten. My younger siblings, who were all under five years of age were on my mother's passport. I have a very clear memory of that.