Replace the question mark with a number

A man and his wife were making their first doctor visit prior to the birth of their first child. After everything checked out, the doctor took a small stamp and stamped the wife's stomach with indelible ink.

The couple was curious about what the stamp was for, so when they got home, he dug out his magnifying glass to try to see what it was. In very tiny letters, the stamp said, "When you can read this, come back and see me."