Louis's grandfather the Prince of Wales attended with the Duchess of Cornwall, with Kate's parents Carole and Michael Middleton also among the guests.
Queen Elizabeth and Prince Philip were not present at the christening of their sixth great-grandchild on Monday afternoon, according to a guest list released by Kensington Palace. Later this week, she will host President Donald Trump for a meeting at Windsor Castle.
William and Kate were aware of Queen Elizabeth's busy schedule several weeks ago.
Kate's sister Pippa is expecting her first child.
The Duchess of Cambridge appeared to make a special gesture to involve her tragic mother-in-law in her grandson's christening.More news: Rescue mission day two : four more boys evacuated
Prince Louis is fifth in line to the throne.
The third child of Prince William and wife Kate was christened Monday at a royal chapel. He was born April 23, 2018.
Van Cutsem is one of four brothers who have been close to William and Harry since they were little.
The Duchess' decision to go back to work so soon will come as a surprise to many, but the royal looked wonderful as she joined her husband and brother-in-law at celebrations to mark the Centenary of Royal Air Force.
A friend said: "One of Harry's happiest childhood memories was being read a bedtime story by his mother". He helped organize William's bachelor party and is married to an American, Holiday Inn heiress Lizzie Wilson. His mother, Lady Carolyn Herbert, was also close friends with their mother, the late Princess Diana.
What will Prince Louis wear?
And like when Prince George and Princess Charlotte, were christened, the couple celebrated by serving slices of fruitcake saved from their wedding to friends and family who attended the event.