A regal slip of the tongue may have let the cat out of the bag about the sex of the latest royal-to-be.
While working a crowd of thousands in the northern English town of Grimsby, the Duchess of Cambridge was given a teddy bear as a gift from a well-wisher.
The duchess said "Thank you, I'll take that for my d...", and stopped herself before going any further, the Telegraph reported.
When the fan asked if she was about to say daughter, she just said: "We're not telling."
The Telegraph said she had claimed not to know the sex of her baby, and historically the sex of royal babies was never announced in advance.
The former Kate Middleton also said on her trip to the port town that her little one was already making its presence felt.
"I asked her if the baby has been moving or kicking," a well-wisher explained to British newspaper The Mirror. "She said 'yes it is, very much so'."
The 31-year-old arrived 70 minutes late after her helicopter was delayed due to fog, but the waiting fans didn't seem to mind.
Crowds gathered to catch a glimpse of the mother-to-be and showered her with gifts for the baby and pretty flowers.
The brunette looked well as she chatted to fans, wearing a brown Hobbs coat over a Great Plains dress.
On her "royal day away", she also told an elderly woman it would be a long time before she took the throne.
Claire Moss-Smith, 86, told the duchess she was waiting for her to become Queen, the Daily Mail reported.
The royal responded: "You might be waiting a long time."
Known for her love of the high street, she has been spotted browsing a Topshop store in London for maternity clothes.
"I saw her in Topshop last Wednesday," revealed Linda, the wife of British politician Austin Mitchell, who helped greet the royal in Grimsby.
"She had handfuls of clothes. When I saw her today I asked her if she bought anything and she said a few bits and bobs. I said I was going to say hello but didn't want to bother her and she said, 'Oh no, that would have been wonderful.'"
Catherine visited the English seaport to perform a series of solo engagements, including visiting Peaks Lane Fire Station and the National Fishing Heritage Centre.
She attended an opening ceremony at Havelock Academy secondary school.
"I've really enjoyed myself. It was a wonderful experience, so authentic - especially the smells and the noises," she thanked staff after the tour of the fishing centre.