Teishi/Sadako was the Emperor's wife already, and she had already given birth to the First Prince, Prince Atsuyasu, whom the Emperor favored, but Michinaga pushed Teishi/Sadako to become an Empress and he daringly implemented the system of One Emperor, Two Empresses.
先立の后に定子がおり、すでに第一皇子・敦康親王を生み帝寵も深かったが、道長は定子を皇后に押し上げることであえて一帝二后を遂行させた。