Duchess of Cambridge reveals she doesn't know baby's sex, but it's a good kicker

Kate, Duchess of Cambridge waves goodbye after watching various sporting activities at The Kensington Leisure Centre. She's about six months pregnant. Photo / AP

The Duchess of Cambridge has revealed she does not know the sex of her unborn child - but it has been kicking.

Kate clutched her stomach as she told a teenager about the phenomenon experienced by pregnant women, during a visit to open a new council leisure centre in Ladbroke Grove, London.

The Kensington Leisure Centre was built on the site of a 1970s pool where the Duke of Cambridge and Prince Harry learned to swim.

In keeping with the theme, the Duchess was presented with a large duck-shaped inflatable yellow ring for Prince George, now almost 18 months old.

Kate is six months pregnant with her second baby, due at the end of April, but looked relaxed as she spent the day in the Royal Borough of Kensington and Chelsea carrying out a series of engagements.

At the new centre, due to open at the beginning of March, the Duchess chatted to groups of children involved in sporting activities from basketball to curling.

Martina McDonagh, 17, who uses a wheelchair as she has spinal problems, quizzed the royal visitor about her unborn child.

The teenager, who attends the Jack Tizard School for youngsters with learning difficulties in nearby White City, revealed: "I said to Kate: 'I hope it's a girl', and she said: 'I can feel it kicking.'

"And she told me she doesn't know if it's a boy or a girl."

William and Harry took lessons at the previous pool on the site when they were pupils at nearby Wetherby Prep School.

George has already started going to the Buckingham Palace swimming pool with his mother but is expected to follow in his father's footsteps by going to the leisure centre when he is a bit older.

-AAP