Will Pippa publish a book next on how to celebrate your birthday when pregnant? The duchess of Cambridge turned 31 on Wednesday and is planning on celebrating “privately,” St. James’s Palace said. A source told Us Weekly that the duchess “wants something low-key” and will most likely spend the day with her parents at home in Bucklebury, although other sources said she will be celebrating in Wales, where she and William live. Catherine is pregnant with the couple’s first child. In Belfast, the Union Jack flew over City Hall to mark her birthday—the first time the flag has flown since the decision not to fly it permanently.