The new royal baby boy's birth date has an important meaning for his family and his country - but his parents already used the name most connected with April 23.
Kate Middleton and Prince William christened their first son George, in 2015. Their third child was born at 11.01am UK time on St George's Day - the feast day for the patron saint of England.
Royal baby names typically have a strong connection to tradition, and many royal watchers have pointed out that George would have been the perfect name.
The unnamed younger brother to Prince George, 4, and Princess Charlotte, 2, will be the Duke of Edinburgh and the Queen's sixth great-grandchild. Bookies have tipped Arthur, Albert, Philip or Frederick as possible names.
It could be a day or two before the name is announced.
Kate and William are likely to leave the hospital in the coming hours.
The newborn's older siblings could first make an appearance at the Lindo Wing of St Mary's Hospital to meet him, just as George visited Charlotte at the London hospital with his father just after she was born.
The baby is fifth in line to the throne, bumping Prince Harry into sixth place, and Charlotte becomes the first royal girl not to be usurped in the pecking order by a younger brother.
St George's cross is on the English flag and forms part of the UK's Union Flag, or Union Jack.
The day is significant for the royal family, with nominations for the Order of the Garter, the highest and oldest order of chivalry in the UK, announced on April 23.
During Royal Ascot week in June, members of the Order of the Garter attend a special ceremony at St George's Chapel in Windsor Castle, where Meghan Markle and Prince Harry are due to be married on May 19.
St George, thought to have been a soldier in the royal army persecuted for his faith, was named protector of the royal family and the patron saint of the country in the Middle Ages.
On Prince George's second birthday in 2015, the Royal Mint released a limited edition commemorative coin featuring the legend of St George and the dragon he killed on one side, and a portrait of George's great-grandmother, the Queen, on the other.
Kensington Palace announced on Monday morning local time that Kate, 36, had given birth in the same place where she had her first two children.
"Her Royal Highness The Duchess of Cambridge was safely delivered of a son at 1101hrs. The baby weighs 8 lbs 7oz [3.8kg]," it said.
"The Duke of Cambridge was present for the birth. The Queen, The Duke of Edinburgh, The Prince of Wales, The Duchess of Cornwall, Prince Harry and members of both families have been informed and are delighted with the news.
"Her Royal Highness and her child are both doing well."