In 2002, with the middle-aged Prince Albert II of Monaco still a bachelor without an eligible heir (previous children born out of wedlock didn't count), Monaco's parliament amended the constitution to allow royal power to pass from a reigning prince with no descendants to his siblings. Parliament was worried, it seemed, that the Prince would remain unmarried and never father an heir. He tackled the first item with his 2011 marriage to former Olympic swimmer Charlene Wittstock, and now is about to check off the latter, twice over.

Her Serene Highness Princess Charlene is due to give birth to twins later this month—the first twins in the royal family history—giving Monaco not one but two eligible heirs. But although they'll share a birthday, the babies won't have equal claim to the throne. As would have been the case if Kate Middleton had given birth to twins and not just baby George, succession follows a strict seniority rule: Whichever baby is born first is the heir. The royal family of Monaco differs from the British monarchy on some of the specifics, though.

British Commonwealth leaders passed a law in 2011 that allowed women born into the royal family the same right to the throne as men, so even if the first twin born was a girl, she would still be the heir. Monaco, however, has made no such amendment and still abides by male priority. So if the twins turn out to be a girl first and a boy second, he gets bumped above his sister in the succession line. The Prince and Princess have abstained from finding out the babies' sexes, so it's not known if this will come up.

There's also the issue of cesarean section, should it prove necessary.

"The obstetrician will always deliver first the twin that presents itself first when the uterus is opened at the time of cesarean section," Dr. Patrick O'Brien, spokesman for Britain's Royal College of Obstetricians and Gynaecologists, told the Associated Press. "We don't decide in advance which twin to deliver first."

Either way, two babies means twice as many gunshots. The palace has already announced that when the twins are born 42 cannon shots will sound, instead of the 21 that would boom for a single baby.