It looks like Kate may have let slip the sex of her baby, due to be born in July.
The Duchess of Cambridge is said to have let her guard down while accepting a teddy bear from a member of the public in Grimsby, almost revealing she was to have a daughter.
She was overheard telling the well-wisher: "Thank you, I will take that for my d..." before stopping herself.
Sandra Cook, 67, who was in the crowd next to Kate, told the Times: "I said to her, 'You were going to say daughter weren't you?' and she said, 'No, we don't know'. I said, 'Oh I think you do', to which she said, 'We're not telling'."
Kate, who is five months pregnant, joked with crowds in Grimsby after arriving more than an hour late.
Kate, whose helicopter from Kensington Palace was delayed due to thick fog, was given flowers and gifts for her baby as thousands turned out to see her.
Her baby bump was visible beneath her chocolate-coloured three-quarter length Hobbs coat, visited a fishing museum, a fire station and a school in the Lincolnshire town.
She also revealed her baby had been kicking, after one onlooker asked if she was feeling her unborn baby move around, to which she replied "Yes, very much so." She is 21 weeks pregnant.
On her way into Peaks Lane fire station, she made an unplanned stop to meet around 10 adults and children who were patients at nearby St Andrew's Hospice.
Stooping slightly to talk to a group of women, who were wrapped in blankets as they sat in their wheelchairs, Kate shared a loud laugh with them.
Claire Moss-Smith, 86, said she had said to the Duchess: "I'm waiting for you to be Queen."
Kate laughed, she said, and replied: "You might be waiting a long time."