When Japan's Princess Sayako married civil servant Yoshiki Kuroda earlier this month, she became the latest royal to conform to a recent trend of marrying non-royals.
Princess Letizia gave birth to a girl, Leonor, in October
A similar thing has been happening in Europe for some time - but the other way round, with princes marrying women without a regal heritage.
Last year, for example, Crown Prince Felipe of Spain married divorced TV journalist Letizia Ortiz.
Before that, Norway's Crown Prince Haakon married single mother Mette-Marit Tjessem Hoiby, and Denmark's Crown Prince Frederik married Australian estate agent Mary Donaldson, who he met in a bar during the Sydney Olympics in 2000.
Royal author Margaret Holder told BBC World Service's Everywoman programme that such marriages seemed to be part of a deliberate strategy to ensure that these monarchies survive.
"All of them have taken in girls as crown princesses who would never have stood a chance even 20 years ago," she said.
"I thought that maybe what has happened is that the kings and queens have sat down and all come to the same conclusion - that the shambles that has befallen us here in Britain will not happen in their countries.
"They have said that their sons will follow their hearts - they will not be made to marry certain members of society, and will not have a virgin bride like Diana. That model will not happen."
All three of the recent royal princesses have subsequently had babies, and the marriages have done much to boost the monarchies involved.
Spanish royals correspondent Jose Costa says marrying "commoners" has given the princes the common touch, and made their families more accessible in the 21st Century.
"We have to remember the Spanish monarchy has been restored after more than 40 years - so it is in many ways a new royal family," he said.
"When the prince, the heir to the throne, married this very well-known divorcee in Spain, I think people found it quite normal."
And he added that the strongly Roman Catholic country immediately warmed to the idea of a former divorcee as a potential queen.
"She's doing very well," he said.
"She's a journalist and she knows how to behave in public. She is beautiful, and now she's a mother - the mother of the future heir. I think she's doing very well."
Meanwhile, Australian journalist Kathy Lette says that although her compatriots were treating the affair with a touch of irony, they were nevertheless in "shock" at having their own member of a royal family - despite the fact that Australia's head of state is the British Queen.
"We think of ourselves as being at the bottom of the biological barrel - in Australia, breeding is something we do with sheep; elocution is how you kill inmates on death row in Texas," she joked.
"So to have a member of the royal family amongst us is completely funny."
Norway's Crown Prince Haakon is seen as a "new man"
But Ms Lette adds that she felt the fuss over Princess Mary did not signal any new fondness for the British Royal Family, as the Australians have "had to put up with the snobbery of the English upper class for so long".
There are estimated to be just under 40 eligible princes around the world - and Ms Lette says the courtship of Frederik and Mary gave some indication of the best way to attempt to get one.
"They met in a bar - which is actually a good place to meet your future prince," she said.
"If you're in the royal family, you're so bored you need someone to liven you up."