I noticed the same thing on my first daughters newborn clothes when I pulled them out for the new baby. I'm guessing that it's actually milk vomit and some of the stomach acid has come up with it perhaps and over time discoloured the fabric?! Just a guess! I just soaked mine in napisan and washed as normal. I hung them out in the sun too, so maybe that had something to do with the stains disappearing... I just couldn't be sure.