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Princess Charlene of Monaco gave birth to twins – Gabriella and Jacques – on Wednesday.
The gender of the twins was kept hidden from even Prince Albert.
Prince Albert II’s father, Rainier III, amended the constitution in 2002 so one of his daughters could inherit if his son - who had fathered two children out of wedlock - failed to produce a legitimate heir.
If the twins were of the same sex then whoever was delivered first would be next in line. Now, however, Jacques will take that role even though Gabriella was born two minutes before him.
"The obstetrician delivered the first twin that presented itself when the uterus opened at the time of caesarean section,” says Dr Patrick O'Brien, spokesman for Britain's Royal College of Obstetricians and Gynaecologists.
Princess Charlene of Monaco, a Zimbabwean-South African ex-Olympic swimmer, married Prince Albert II in 2011.