Although the new princess is unlikely to ever become queen, it is not unheard of for a second child in the direct line of succession to accede to the throne.
Mostly recently the Queen's own father, King George VI, replaced his elder brother, King Edward VIII, who abdicated over his love for the "unsuitable" American divorcee Wallis Simpson.
Thanks to a recent change in the law, the couple's daughter cannot be superseded in her claim to the throne by any younger brother who may yet come along.
The Succession to the Crown Act, passed in 2013, means that male heirs no longer precede their elder sisters in the line of succession. A 2012 intervention by the Queen also means that the new baby will automatically become an HRH - Her Royal Highness - and be permitted to take the title of princess.
As the second child of the second heir to the throne she would, until then, only have been a Lady.
Her arrival will shunt her uncle, Prince Harry, down to fifth in the order of succession. Her great uncle, Prince Andrew, will fall out of the top five for the first time to sixth.
Many royal watchers had been desperately hoping for a girl - the last to be born in the direct line of succession was Princess Anne back in 1950.
Both Prince Charles and William's late mother, Diana, Princess of Wales, both openly longed for a girl to add to their family.
Indeed, when congratulated by a member of the public about the news that he would become a grandfather again last year, he said candidly: "It's wonderful to be a grandfather again. I'm looking forward to it. But I hope it will be a girl this time."
Kate herself has said William was hoping for a daughter, although his brother, Prince Harry, joked William probably doesn't realise what he was taking on.
News of her birth came in the form of a press release to media organisations followed by a formal - and very modern - announcement two minutes later on the palace social media channels.
However, it was made public only after the Queen and other family members had been informed, including Prince Charles, now a grandfather twice over, the baby's uncle Prince Harry, who is currently on an army detachment in Australia, and, of course, Kate's parents, Michael and Carole Middleton, who will play a dominant part in their new grandchild's life.
Here's a look at some younger royals.
PRINCE HARRY (born 1984)
The second son of Prince Charles and Diana, Harry is often seen as the mischievous one, the fun-loving counterpart to the more staid - some say dull - William.
Harry came of age under full public scrutiny, and through the years he has sparked some scandalous headlines. He admitted to smoking cannabis and drinking in his teenage years, and in 2004 he was photographed scuffling with a photographer outside a London nightclub.
A couple of incidents were particularly embarrassing for the royals: When the prince was photographed wearing a Nazi-themed costume to a fancy dress party, prompting the headline "Harry the Nazi," and more recently when he was pictured partying naked in Las Vegas.
Like many other royals, Harry chose a military career and has served in Afghanistan. That will likely continue to be his main role as he gets bumped further down the line of succession. The new royal baby will see Harry relegated to fifth in line.
PRINCE ANDREW (born 1960)
Andrew, the queen's second son and Charles' younger brother, gets more press than fellow siblings Anne and Edward - largely for the wrong reasons.
Andrew, the Duke of York, enjoyed a successful naval career as a helicopter pilot and served in the Falklands War, yet that record has been overshadowed in recent years by headlines about his friendship with several controversial figures, notably U.S. financier Jeffrey Epstein, a registered sex offender. Andrew stepped down from his role as a trade envoy in 2011 as questions mounted, and this year he had to publicly deny claims that he had sex with an underage woman.
Andrew's chaotic marriage to Sarah Ferguson - known as Fergie - ended in divorce. He has long been criticized for his opulent, globe-trotting lifestyle, and his romantic links to a number of models and starlets have attracted unwelcome nicknames like "Randy Andy."
Andrew will become sixth in line to the throne with the baby's birth.
PRINCESS MARGARET (1930 -2002)
Four years younger than the queen, Margaret was Elizabeth's only sister. With her film-star looks and vivacious personality, Margaret lived a glamorous life, and many remember her best for her turbulent romances.
The princess's relationship with divorced pilot Peter Townsend was frowned upon by Winston Churchill and the Church of England, among others. In 1955, aged 25, she declared she had decided against marrying him, "conscious of my duty to the Commonwealth."
Margaret later married photographer Antony Armstrong-Jones, a commoner, and the couple became the heart of a fashionable set in the Swinging London scene of the '60s and '70s. The princess was often snapped dancing late into the night, threw famous parties in the Caribbean and mixed with pop stars like Mick Jagger.
Before the couple divorced, Margaret met Roddy Llewellyn, 17 years her junior, a relationship that prompted huge media coverage.
Margaret's health declined in her 60s, and she died in 2002 at 71.
KING GEORGE VI (1895-1952)
The father of the queen, George VI - born Albert - became the unexpected king when his elder brother, Edward VIII, abdicated in 1936 after a reign that lasted just 11 months. Edward, often portrayed as a raffish playboy, had abandoned the throne to marry his mistress, the divorced American socialite Wallis Simpson.
A shy man with a stammer, George had to restore public faith in the monarchy and be the symbolic leader of a country at war with Germany. The Oscar-winning film "The King's Speech," which dramatized the story of how he overcame his initial struggles as monarch, reignited interest in his often overlooked life.
George died at 56 in 1952.
This is how the succession line will now look.
1. The Prince of Wales
2. The Duke of Cambridge
3. Prince George
4. The new royal baby
5. Prince Harry
6. The Duke of York