Fetal dynastic succession

Imagine the following scenario: a king in a patrilineal dynasty dies without a male heir. Therefore the crown passes to either his eldest daughter or to his closest male relative, and this person is installed as the new queen or king. It is subsequently discovered that the queen dowager is pregnant by the late king with a male child. What happens to the new monarch, then? Do they have to abdicate in favour of the child once it is born, or does they continue to reign until their natural death, after which the throne would pass to the queen dowager’s son?

I suppose answers will vary according to the rules of succession of a particular house, so feel free to answer for any house whose rules you may be familiar with.

Civil wars have been fought over less.

IIRC, you are describing exactly the situation that happened with King Alfonso XIII of Spain.

This is a perenially popular question about monarchy, and it describes a situation which has happened twice in European history. In both cases, an interregnum followed while the kingdom waited to ascertain the sex of the unborn child, and upon being male, the newborn immediately became king. The two cases were John I of France in 1316 (who unfortunately died a week later) and Alfonso XIII of Spain in 1886.

This still doesn’t answer all of my question, though. In my scenario, there was a reigning monarch from the time the king died until at least the time the pregnancy was discovered. What happens when the pregnancy is discovered? Does the monarch abdicate, thus effecting the interregnum shortly after the pregnancy is discovered, or is the monarch’s installation anulled, thus effecting the interregnum retroactively from the moment of the king’s death?

There probably wouldn’t be a new monarch crowned until it was certain that the queen was not pregnant (or, if she was pregnant, that the baby would not be a son). A regent would probably be appointed for the interregnum (and until the child was old enough to rule, if it turned out to be a boy).

There shouldn’t be, though. If there is any reasonable possibility that the queen might be pregnant, the throne should be “in abeyance” under a regency until she is certain.

Granted, in real life that might not happen, especially in earlier times when monarchs had real power and a grasping younger brother or daughter might seize the throne at once. But if you’re playing by the rules, you should wait on a potential unborn child, and if you make an honest mistake, you should retract it as cleanly as possible.

A new monarch does not, technically, accede to the throne until it’s certain that he or she is the monarch. In point of fact, what happens is that a regent is named. (In point of fact, there have been sub rosa comments that this is precisely what happened with regard to Elizabeth II – she took the throne as heir presumptive of George VI, not heir apparent, and it is said that Elizabeth (consort of George VI and known as the Queen Mother for the next 50 years) had not in fact reached menopause, so that if George VI had begotten a son before his death, he would be monarch from birth instead of his 26-year-old married sister.