[paulific]

Notably there have only been two reigning empresses since the 8th Century and none at all in the modern era. The current Imperial Household Law only allows males to ascend the throne, so they were in real trouble 10 or 15 years back before the birth of the current Emperor's nephew who is now 2nd in line to the throne. The succession is not fixed by the constitution, so they could change the law any time they like. But they probably won't unless they have to.