The Duke and Duchess of Cambridge's baby will be born third in line to the throne and a great-grandchild to the Queen.
The last time a still-serving monarch got to meet a great grandchild born in direct succession to the crown was nearly 120 years ago.
Queen Victoria, who reigned until 1901, was still sovereign when her great grandchild Edward VIII, who later abdicated, was born third in line in 1894.
The Duke and Duchess of Cambridge's baby will be born third in line to the throne
His brother George VI was also born in Queen Victoria's lifetime, arriving fourth in line in 1895.
The last great granddaughter of a still-serving sovereign born in direct succession on the male line was their sister Princess Mary in 1897.
William and Kate's baby - a future King or Queen - will be the great great great great great grandchild of Queen Victoria.
When the newest addition to the Royal Family arrives, Prince Harry, the baby's uncle, will be bumped down the line of succession to fourth place.
The Queen has already welcomed her first great grandchild, Savannah Phillips - the daughter of Peter and Autumn Phillips - but Miss Phillips is only 12th in line, being born down the female line as a granddaughter to the Princess Royal. Her second great grandchild - Savannah's sister - Isla was born in March 2012, 13th in line to the throne.
Royal births are usually celebrated with a Royal Salute of 41 guns.
They are registered in the normal way, although the Home Secretary is required to notify certain officials including the Lord Mayor of London, while the Queen's Private Secretary Sir Christopher Geidt informs Governor Generals overseas.
There is also the age old custom of attaching Notice of Royal births and deaths to the railings at Buckingham Palace for members of the public to read.
But as well as keeping to this tradition, the arrival of royal birth will no doubt also be officially declared via the monarchy's websites, on Facebook and via Twitter.