What Is Conceived in Mary, Is of the Holy Ghost?

Joseph decided to put Mary away, privily, but an angel of the Lord appeared in a dream and told him he had to marry her. Matthew's Gospel says "that which is conceived in her is of the Holy Ghost" Some people wonder why it would say things like that; we don't know what original version said. It could be one of those plain and precious truths that gets slightly modified because copyists don't quite know how to interact with it.