An heir presumptive's position may not even be secure after they ascend their throne, as a posthumous child of the previous monarch could have a superseding claim.
Following the death of William IV in 1837, he was succeeded by his niece Queen Victoria, whose accession proclamation noted her accession was only permanent so long as a child of William was not born in the following months to his widow, Adelaide, even though Adelaide was 44 years old and had last been pregnant 17 years earlier.
[3][4] Such a situation occurred in Spain in 1885, when King Alfonso XII died and left behind a widow who was three months pregnant.
In some monarchies, the heir apparent bears, ipso facto, a specific title and rank (e.g., Denmark, Netherlands, United Kingdom), this also sometimes being the case for noble titleholders (e.g., Spain, United Kingdom), but the heir presumptive does not bear that title.
The list is limited to heirs presumptive who did not succeed due to death, abolition of monarchies, or change in succession law.