Last October, the Commonwealth proposed a revision to the royal laws regarding succession in order to provide gender equality -- i.e., a first-born daughter would ascend to the crown in favor of a younger male sibling. (If the current Queen Elizabeth II had a younger brother, he would have become king when her father King George VI died in 1952. Reportedly, Elizabeth herself cheered the new rule.)

“Put simply, if the Duke and Duchess of Cambridge were to have a little girl, that girl would one day be our queen,” Prime Minister David Cameron said.

(The Commonwealth also proposed that a British monarch could marry a Roman Catholic.)

Thus, under the new rules, if William and Kate, the Duke and Duchess of Cambridge, give birth to a daughter, she would theoretically ascend to the throne upon the death or abdication of William (assuming he becomes the king one day).

But, what if Kate produces a pair of twins?

That blessed event, which is a distinct possibility given Kate’s age, 30, and her acute morning sickness [hyperemesis gravidarum], would raise some intriguing possibilities.

Twins, identical or otherwise, do not exit the mother’s womb simultaneously -- one is born seconds before the other. And those few seconds would determine which twin would one day become king or queen.

To put it bluntly, whichever baby comes out first will be the monarch.

Of course, if complications from Kate’s pregnancy result in some kind of emergency, perhaps one that requires a caesarean section, that would confound matters. In that case, the royal obstetrician Alan Farthing may inadvertently have the power of choosing Britain’s future monarch.

(As a bizarre aside, Farthing was the former fiancé of Jill Dando, the BBC broadcaster who was murdered in 1999.)

There is a precedence for this scenario -- in January 2011, the Crown Princess of Denmark, Mary, and her husband, Prince Frederik, gave birth to twins, Josephine and Vincent. But the two Danish babies have elder siblings, Christian and Isabella, meaning they may never reach the throne. (For those who are counting, Vincent came out about half-hour ahead of his twin sister.)

The last time a British monarch produced twins was in 1430, when Queen Joan Beaufort, the wife of King James I of Scotland, gave birth to two boys -- the first-born, Alexander Stewart, was named the heir to the throne. However, Alexander died less than one year later and his slightly younger brother, also named James, took his place. By the time James turned six, his father was murdered, and the boy eventually became King James II of Scotland (who was, incidentally, the last Catholic monarch in Britain).

Other European monarchs have also produced twins. In 1556, Catherine de Médici, the Queen consort of France and wife of King Henry II, gave birth to twin baby girls, Joan of Valois and Victoria of Valois. Joan died during childbirth; Victoria died a month later. They were the last children Catherine would ever have.

Then 150 years later, another Queen Consort of France, Maria Leszczyńska, daughter of King Stanisław Leszczyński of Poland and wife King Louis XV of France, gave birth to twin girls, Marie Louise Élisabeth and Henriette Anne.

Both girls would die of smallpox as young women.

One can only hope Kate’s offspring have much happier outcomes.