Primogeniture for the unborn

[Last Hussar]

The succession only goes back up the tree if it hits a dead end

[grahambeyrout]

Victoria's succession after William IV was conditional on whether or not Williams widow was pregnant. It she had have been Victoria would have to give up the throne. Nowadays of course it is easy to test for pregnancy, presumably there would be a regent appointed.

[Last Hussar]

That was written into the succession, wasn't it, as a one off.

The Henry-Mary-Edward thing is different because the gender made the difference.