Here are 10 facts about royal births.
1. A team of 23 medical staff was on hand for the birth of Prince George and Princess Charlotte at the private Lindo Wing.
A handful of midwives and others led by a consultant obstetrician were in the delivery room, but obstetricians, gynaecologists, surgeons, haematologists and theatre staff were also waiting in the wings in case of an emergency.
Third time mums like Kate need to be watchful
2. After William was born in 1982, the Prince of Wales wrote how he was "so thankful I was beside Diana's bedside the whole time".
"I really felt as though I'd shared deeply in the process of birth," he added.
All you need to know about the royal baby
3. Diana was induced because she could not bear the pressure from the media any longer - and claimed doctors had to find a date that suited Charles and his polo fixtures.
4. The Queen, then Princess Elizabeth, was given an anaesthetic to help with the pain while in labour with first child Charles in 1948.
5. A restless Duke of Edinburgh occupied himself by playing squash while awaiting the arrival of his firstborn.
6. The Queen had all her four children - Charles, Anne, Andrew and Edward - at home at Buckingham Palace and Clarence House.
7. Princess Elizabeth was born at home by Caesarean section in her grandparents' house in Mayfair, London.
She was breech and it was a difficult birth for her mother the Duchess of York.
Will it be a boy or a girl?
8. It used to be the custom that government ministers and other witnesses were present at royal births to ensure no substitute child had been smuggled in in a warming pan or similar receptacle.
9. But Queen Victoria put her foot down when her great-grandchild, the future Edward VIII, was born in 1894 and declared that just one Cabinet minister would be needed, with only the home secretary attending from then on.
10. The birth of the Queen's cousin Princess Alexandra on Christmas Day in 1936 was the last occasion that a home secretary was present, meaning the Duchess of Cambridge has been spared such an intrusion.
Prince Charles's birth was the first time in centuries that there was not a government minister there to witness the arrival of a future heir to the throne.