London - Speculation about baby names, hair colour and hypnosis swirled around Britain's Duchess of Cambridge on Thursday during her last solo public appearance before she gives birth to a royal heir.

British glossy magazine Grazia reported the duchess has considered using hypno-birth for a delivery expected next month in which women use a form of self-hypnosis to control pain by learning techniques to boost production of pain-relieving hormones.

Bookmakers said bets favoured the duchess joining the growing band of women dubbed "too posh to push" by opting for a caesarian rather than a natural birth.

The royal family has declined to give any details on plans for the first child of Prince William and Kate Middleton, 31, who have been officially known as the Duke and Duchess of Cambridge since their marriage in April 2011.

A royal spokeswoman said she was aware of rising speculation about the birth but declined to confirm or deny any details for the baby who will be the third in line to the British throne after Prince Charles and his oldest son William.

"This is a happy occasion and there is lot of speculation about what the duchess might and might not do but this really is a private matter," the spokeswoman said.

Royal watchers expect the baby to be born at St Mary's Hospital in central London where Princess Diana gave birth to William, who turns 31 next week, and his brother Prince Harry.

The duchess was under scrutiny for any hints about the baby during her last solo public engagement on Thursday when she launched Princess Cruises' new 3,600-passenger vessel Royal Princess in Southampton on England's south coast.

BABY TALK

Wearing a black hat and Dalmatian print coat, the duchess was careful to give nothing away after a slip in March when she accepted a teddy saying: "Thank you, I'll take this for my d.."

Rupert Adams from bookmaker William Hill said this dropped "d" led to such wide speculation that the baby was a girl that it suspended all bets on gender just weeks later while rival bookmaker Paddy Power paid out on bets that it was girl.

Bets on names have continued to roll in with Alexandra the clear favourite followed by Diana, after Prince William's mother who died in a car crash in 1997, and then Elizabeth.

Alexandra has a history in the royal family with Princess Alexandra, 76, a cousin of Queen Elizabeth and Queen Victoria's son Edward VII married to Alexandra of Denmark in 1863.

The favourite boy's name is George and the favoured date for the birth is July 17.

"We are seeing lots of small bets from over 100 countries. The Aussies and Canadians particularly love this," said Adams.

Rory Scott from Paddy Power said people were also putting money on more obscure bets, such as the baby's future career, university, and hair colour with brown favoured over ginger.

"You combine the royal family with betting and you have two of the favourite pastimes in Britain," Scott said.

Both bookmakers found gamblers expected the duchess to opt for a caesarian instead of natural birth with one in four babies now delivered this way in the UK, up from 12 per cent in 1990.

The focus on how the duchess gives birth has drawn her, unwittingly, into the complex web of childbirth politics where campaigners of different birthing methods battle for supremacy.

The Blissful Birth website which deals with hypno-birthing said a number of hypno-birthing practitioners have "spun" speculation about the duchess using hypno-birthing into suggestions that she was considering their particular approach.

But the website's founder Heidi Woodgate, urged these practitioners to stop the race for recognition.

"Quite frankly, whether my customers are royalty, celebrities or normal everyday women, the focus should be on their birth story, not the particular hypno-birthing programme they used," Woodgate wrote in a blog on the website.