Elsewhere, in the Benelux countries, such a situation has been prevented by the abdications of reigning monarchs retiring and handing down their thrones to mature heirs in the prime of life, ready to assume the duties and responsibilities of sovereignty before advancing too far in age themselves.
But at the opposite end of the matter, in the distant past we have observed numerous accessions of young -- sometimes even minor -- sovereigns. Oftentimes this happened because royal succession skipped a generation -- or two, as happened in France, where King Louis XIV was succeeded by a great-grandson, King Louis XV.
Given the vicissitudes of history, it's no surprise that fluke factors frequently played into royal succession. Neither Maximilian II of Bavaria nor Leopold III of the Belgians, although heirs-apparent (meaning crown princes due to eventually inherit thrones), expected to succeed when they did. Both became kings early through unexpected developments (abdication and accidental death).
Nobody knows if and when the man we know in history as Czar Alexander III of Russia (who died in 1894 at the age of 49, having reigned for only 13 years) would have succeeded, but for the assassination of his father, Czar Alexander II. As it was, he hadn't finished fathering children when he came to the throne: his youngest child (a daughter named Olga) was born in 1882, the year of his coronation.
His consort, Czarina Maria Feodorovna, was 34 years old at the time and could easily have found herself in the situation of being a pregnant new queen the preceding year, at the time of her husband's accession. After all, her previous child (Grand Duke Michael Alexandrovich) had been born in 1878.
The woman we know in history as Queen Elizabeth, the Queen Mother of the United Kingdom, could also have found herself in that situation. She, too, was well within childbearing age (36) at the time of her own husband's unexpected accession to the British throne. And she had barely passed it, when in 1952 she became a widow: a pregnancy test had to be conducted, to confirm the status of her elder daughter, Elizabeth II, as the new queen regnant.
Earlier in royal history, Dowager Queen Adelaide -- another woman who could theoretically have been pregnant at the time of accession -- had to also be confirmed not to be pregnant when her husband (King William IV) died (1837). This was to confirm the accession of his niece Victoria as queen regnant.
As it was, there was no doubt that Dowager Queen Maria Cristina of Spain, widow of King Alfonso XII, was pregnant at the time of her husband's death in 1885. In that case, it was simply about waiting for the birth of her third child, whose sex would determine the status of her elder daughter (Infanta Maria de las Mercedes) in the country. As it was, the posthumous child was a boy: to the best of my knowledge, Alfonso XIII of Spain has been the only person in history who was king from his mother's womb.
That being said, have there been other pregnant dowagers in royal history? If I understand correctly, Queen Astrid of the Belgians was expecting her fourth child, as of her tragic death in a car accident in 1935. Had fate decreed otherwise, she might have survived while her husband lived, in which case she would have been a pregnant dowager. Of course, it's not likely that her unborn child would have survived the accident ...
Speaking of Astrid: she and Queen Marie of Bavaria had something else in common: both were Protestant-born princesses who married Catholic crown princes, and eventually converted to the faith themselves.
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