According to the story, Madame Valamonde and Armand noticed the baby's dark skin several weeks before Desiree. Can you offer a possible explanation for this gradual increase in pigmentation over the course of three months?

I don't believe that this was the increase of pigmentation, as it was that the initial excitement of childbirth had begun to fade. That and the fact that outsiders were now visiting..... people who didn't have their feelings overshadowed by love.