Which number replaces the qu...

A couple was making their first doctors visit prior to the birth of their first child. After the exam, the doctor took a small stamp and stamped the wife’s stomach with indelible ink. The man and his wife were curious about what the stamp was for, so when they got home, the man took out his magnifying glass to try to see what is was. In very small letters, the stamp said, “When you can read this, come back and see me.”