Why Is the Michiko called 上皇后 and not 皇太后?

Question

I feel the answer to this might be super simple, like the fact that her husband is still alive and holds the title of 太上天皇. But I just want clarification on this.

Should Akihito pass away, then would Michiko's title change to 皇太后 or would her title stay as 上皇后?

I understand that 皇太后 = dowager empress, and that the English translation of "dowager" means that the title would go to an empress whose husband has passed away, but is this really the case in terms of Japanese imperial titles or is there some other formalities that the English translations don't capture?

Are these correct?

• 太上天皇 = retired emperor
• 上皇后 = retired empress
• 皇太后 = dowager empress

Answer 1

Traditionally a 皇太后 is (roughly) either a mother of emperor or a wife of a former emperor. So the current 上皇后 could be called a 皇太后, but is not due to the law.

Wikipedia::皇太后

なお、2017年（平成29年）6月16日に公布され[2]、2019年（平成31年）4月30日に施行された[3]、天皇の退位等に関する皇室典範特例法では、第3条で「前条の規定により退位した天皇は、上皇とする。」、4条で「上皇の后は、上皇后とする。」とそれぞれ規定しており、歴史上、太上天皇の配偶者にも用いられてきた「皇太后」称号を使用せず、歴史上、一度も存在しなかった「上皇后（じょうこうごう）」という称号を定めており、議論を呼んでいる。

So apparently 上皇后 was coined quite recently.

The article contains how 皇太后 meant differently in different times. Note that until the end of 平成, there had been no 上皇(太上天皇) = retired emperor in the first place so that the word was not exactly common outside contexts of premodern history.