Martina, a wheelchair-bound pupil at Jack Tizard School in White City met Kate, 33, when the six months pregnant future Queen officially opened Kensington Leisure Centre in west London.
Martina told reporters: "I told her I hoped it was a girl but she said she doesn't know yet. She said it was moving around a lot."
Even before the Duchess's new baby is born, he or she has already got free lifetime membership of the £25 million leisure centre, which has three swimming pools, a 120-station gym, an eight-court multi-use hall, and dance studio.
Kensington and Chelsea Council presented the Duchess with life membership for all the family, along with a duck-shaped rubber ring for Prince George and his own monickered kit bag for swimming lessons.
Princes William and Harry both learned to swim in a previous incarnation of the leisure centre on the same site when they were pupils at nearby Wetherby Prep School.George, who is almost 18 months old, has already started going to the Buckingham Palace swimming pool with his mum but is expected to follow in his father's footsteps by going to the leisure centre when he's older.
Tim Ahern, the council's Cabinet member for environment and leisure, said: "The Duchess was talking about Prince William learning to swim here and she said she hoped Prince George can come here to learn too.
"If George goes to school at Wetherby or one of the other local schools, he will almost certainly come here. Wetherby will use it. All the local private and council primary schools. Book all the lessons that we have."
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