Irregular

"O Wisdom, O Holy Word of God,
  you govern all creation with your strong yet tender care. 
  Come and show your people the way to salvation."
One of the themes that sticks out for me in Matthew's genealogy of Jesus (1:1-15) is the irregular appearance of four women in the list.  Matthew uses the same pattern as he goes from generation to generation, but he breaks it up with the mention of four women: Tamar, Rahab, Ruth, and Bathsheba.
Tamar disguised herself as a harlot to conceive sons by her father-in-law.  Rahab was a harlot of Jericho who aided Joshua's spies.  Ruth insinuated herself into Boaz' life after being widowed.  Bathsheba was the neighbor's wife with whom David committed adultery and fathered Solomon. 
Their inclusion in the genealogy of Jesus prepares for and foreshadows his irregular birth.  It also helps us to see that God can use the most surprising people for his purposes, even you and me.