However, the decision for them not to attend Louis' baptism at St. James's Palace in London was understood to have been mutually agreed by the sovereign and the Duke and Duchess of Cambridge some time ago, and was not health-related.
Members of the royal family congregated at the private ceremony on Monday afternoon including siblings Prince George and Princess Charlotte, Prince Charles and the Duchess of Cornwall.
The Queen, 92, has a busy week ahead, with high-profile celebrations in central London marking the centenary of the RAF on Tuesday and a visit by US President Donald Trump in Windsor on Friday.
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From the royal family, the baptism was attended by William's father Prince Charles and his wife Camilla; William's brother Prince Harry and his new wife Meghan; plus Louis' siblings George and Charlotte.
There were 30,000 copies in the first run of AA Milne's classic tale of the lovable bear and his friends in Ashdown Forest, East Sussex.
This is the first time the family of five has been seen as a full unit in public, People reported.
Kate's brother James Middleton, and Kate's pregnant sister Pippa with her husband James Matthews, the brother of former Made In Chelsea star Spencer Matthews, were also among the guests.
Louis, who was born April 23 in London, will be officially baptized into the Church of England in the traditional christening ceremony on July 9 at the Chapel Royal in St. James's Palace, also in London.