The child will be third in line to the throne behind Prince Charles and Prince William. Prince Harry will now be bumped down to fourth. The laws of succession were recently changed, meaning a daughter would not be bumped by the arrival of a little brother.
William and Kate have been tight-lipped about potential names -- so far. It is believed they have drawn up a shortlist but are waiting for the baby's arrival before making a final decision.
What we do know for sure is the baby will be an HRH and a first Prince or Princess of Cambridge. The Queen ruled in January a daughter would be a Princess - under previous convention a daughter would have been titled Lady, although a son would have been a Prince.
Charles and Diana waited a week before announcing their son would be called William, Prince Andrew and Fergie took two weeks over Princess Beatrice and the Queen took a whole MONTH before giving Prince Charles his name.
The Palace insisted the couple didn't know whether they were having a son or a daughter.
The world learned of the baby's birth in the same way the births of generations of royals have been announced, with a note on the front gate of Buckingham Palace. It was the same way we learned about William's birth. The easel hasn't been used since it announced William's arrival in 1982.
The Queen, senior members of the royal family and the Middleton family - if they were not at the hospital - were told about the birth first. Then a royal aide hurried from the hospital to the palace under police escort with a bulletin. The note, bearing the Buckingham Palace letterhead and signed by key medical staff, was later displayed to the word.
Obstetrician Marcus Setchell, who has been gynaecologist to the Queen since 1990, delivered the baby after delaying his retirement.
Kate gave birth in the Lindo wing of St Mary's Hospital in Paddington.
Kate went on maternity leave on June 13.
Prince William is following modern convention by taking paternity leave. He is expected to take the two weeks paid leave offered by the Ministry of Defence. He will then return to his job as an RAF search and rescue pilot. Statutory paternity leave has only been in force in the UK since 2003.
The birth was in a hospital. While it might seem obvious that the Duke and Duchess of Cambridge's baby would be born in a hospital, Prince William was actually the first would-be king to arrive in such a way. Both he and his brother Prince Harry were born in the private Lindo Wing of St Mary's Hospital in Paddington, west London, where the duchess will give birth. The Queen gave birth to Prince Charles and Prince Andrew at Buckingham Palace.
In January, a statement from Buckingham Palace confirmed the couple's pregnancy to the world: "Their Royal Highnesses, The Duke and Duchess of Cambridge, are delighted to confirm they are expecting a baby in July."
In December 2012, Kate was rushed into the King Edward VII hospital with severe morning sickness, early in the pregnancy.
The birth due date fell in the middle of the Coronation Festival, a four-day celebration to mark 60 years since the Queen was crowned, 16 months after being proclaimed Monarch on her father's death.
They were wrong! According to two of Great Britain's most popular betting sites - Ladbrokes and Paddy Power - the royal couple were going to have a little girl and will name her Alexandra.
The odds are 2-to-1 the royal baby will have brown hair like his mother, not the golden tresses of her dad or her Uncle Harry's fiery red, which are both longshots at 11-to-4.
According to a royal historian, William will be the first-ever father in the delivery room for a royal birth.
New mother Kate is older than Prince William. She was born on 9 January, 1982. Prince William was born on 21 June, 1982.
Kate will reportedly break with tradition and raise her child without the help of a royal nanny.
Under protocol, the Foreign Office still informed all British missions around the world about the royal birth by diplomatic telegram. British diplomat Dominick Chilcott told The Washington Post: "In the days before the 24/7 media, far-flung diplomatic missions might have got their news first by diplomatic telegram, depending how well they could tune their radios to the BBC World Service short wave signal. And especially in Commonwealth countries and other places with close connections to the Royal Family, it would have been an important duty to inform the local government about the birth of a royal baby in line to the throne. Diplomatic telegrams these days are very different from how they were even when I joined the Diplomatic Service 30 years ago. Then, they were the only form of rapid written communication and involved staff at either end sending and receiving the message. Now, they're really just a particular kind of email."
The christening robe will be a replica of one that has been used since 1841.
Analysts predict the arrival of the royal baby could boost Britain's economy by $380 million.
The baby will fall under the zodiac sign of Cancer, just like Prince William, who was born on June 21, and his mother, the late Princess Diana, who was born on July 1.
The due date was believed to be July 13, although the Palace never officially announced it.