The Prince, 65, was asked about the pregnancy during a Prince's Trust event in South London on Tuesday.
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Prince Charles has said he hopes his second grandchild is a little girl
Corey Johnson was among those waiting to shake hands with the royal during his visit to Bermondsey. He congratulated Charles on becoming a grandfather, to which the Prince cheerfully replied: "I already am!"
He added: "But it's wonderful to have another one. I'm happy I'm going to be a grandfather again. I'm looking forward to it – but I hope it will be a girl this time."
Corey then replied, "That's a bit biased", promoting laughter from Charles.
William and Kate's new arrival is thought to be due in April or May
William and Kate's second child is thought to be due in April or May – with the Queen's birthday falling on 21 April, Elizabeth is already the bookies' favourite for a baby girl.
According to the Telegraph, the Duchess, 32, has told friends that she wants to have "at least" three children before the age of 35.
Kate herself is the eldest of three, and is said to want to give Prince George and his siblings the same sort of family environment she grew up in.
The Duchess of Cambridge is said to want "at least" three children
She is just 20 months older than her sister Pippa and less than five years older than brother James, and remains incredibly close to both; Prince George and his younger brother or sister will have almost exactly the same age gap as Kate and Pippa.
On the day that her pregnancy was announced, the Duchess was forced to pull out of an engagement after suffering with the same severe morning sickness she endured during her first pregnancy.
William, 32, is reported to have said that his wife had been ill for about a week, but said that they were both "thrilled" by the news.