Thankfully for the Duke and Duchess of Cambridge, Louis was very well behaved during his eagerly anticipated christening, and the 11-week-old stayed fast asleep as his mum carried him into the chapel.
The Queen, 92, was travelling back from Norfolk on Monday and has a busy week ahead, with celebrations marking the centenary of the RAF on Tuesday and US President Donald Trump's visit to Windsor on Friday. Prince George, who will turn five next week, and Princess Charlotte, three, looked adorable as always as they accompanied their parents and new sibling to the private service.
The Queen and Prince Philip were not in attendance.
According to body language expert Judi James, Meghan deliberately toned down her behaviour to avoid drawing any attention away from the Duke and Duchess of Cambridge.
DOMINIC LIPINSKI/PA The Duchess of Cambridge carries Prince Louis.
It is a replica of the dress made for Queen Victoria's eldest daughter in 1841.
It marks a rare public appearance for Kate during her six months maternity leave, who has only been pictured on official royal business three times since giving birth to her third child.
In one photo, Prince George and Princess Charlotte can be seen pressing their faces to the window, watching the event unfold.
Kate wore a V-neck dress with puff sleeves that fashion trackers at What Kate Wore identified as being from Alexander McQueen, her go-to fashion house.
His Royal Highness Prince Louis Arthur Charles of Cambridge will follow in his brother and sister's footsteps by wearing a christening gown used for every generation of royal infants.
The occasion took place at the Chapel Royal, St James' Palace.
They were served slices of cake taken from a tier of William and Catherine's wedding cake, the Palace said.
Next is Hannah Carter, an old friend of Kate Middleton's from Marlborough College who is married to Robert Carter.