Whether wrapped in pink or blue, the new baby of Kate, Duchess of Cambridge, will have the power to influence trends across the globe as the latest addition to a powerful fashion dynasty.

Whenever Kate or her baby Prince George wear items, they rapidly sell out, in a marketing phenomenon dubbed the "Kate effect" that is helped along by social media and online shopping trends.

Kate's maternity wear and Prince George's baby clothes have proved particularly popular.

"They have global clout," said Geraldine Wharry, head of trend analysis firm Trend Atelier.

"Everyone is going to be waiting to see what the baby is going to wear and what she's going to look like post-pregnancy," she said.

Social media allows fans to track down and share exactly the items that Kate is pictured wearing -- and it helps that she mixes expensive designer wear with more affordable high street brands.

- Selling a fairy tale -

Four years ago Susan Kelley set up WhatKateWore.com, a site that publishes links to online retailers where readers can buy exact clothes worn by Kate.

"It just exploded," Kelley said: the site can receive almost a million page views a month, largely from the English-speaking world but also from countries as varied as the Czech Republic and Libya.

"More than anything it stems from wanting to believe in a fairy tale," Kelley said -- that the middle-class Kate grew up to marry a prince. "It's not another terrorist attack, it's not something horrible... It's something that's fun."

The handful of photos released of Prince George have been analysed on WhatPrinceGeorgeWore.com -- set to become WhatKatesKidsWore.com when the new baby arrives.

"If it's a girl the interest will be extraordinarily high," said Kelley. "There seems to be strong sentiment that as enjoyable as little boys are, a baby girl is even more fun".

The British monarchy has been estimated as having a brand value £53.6 billion ($80 billion, 74.5 billion euros) by valuation consultancy Brand Finance.

Though Prince George has yet to reach his second birthday, he can claim part of that influence.

British brand Cath Kidston is considering re-issuing a soldier-print jumper he was pictured wearing "due to popular demand", a spokesperson said.

George has helped inspire a "nautical theme" due to continue in childrenswear, according to Catherine Hudson, beauty and fashion editor of Prima Baby magazine.

"Kate has always dressed George in classic fashion pieces, so although we may see her trial some of these trends, I think she will mainly stick to traditional silhouettes - whether it is a boy or a girl - and, hopefully, continue supporting great British design," Hudson said.

"If it is a girl, I predict we might see some of her favourite fashion designers bring out a girlswear collection."

- Sold out in minutes -

Brands are keenly aware of the value of being associated with the family, some paying commission if WhatKateWore.com drives traffic to them, and the site has received more requests from advertisers than it has space to host.

Particular hit items have been Kate's nude court shoes by LK Bennett, dresses by the ethical London brand Beulah, and a striped nautical top by ME+EM.

Kate has even been credited with driving a trend of modesty in maternity wear. Items she wore from British brand Seraphine all sold out, and her patronage significantly boosted the little brand's rapid expansion, a spokeswoman told AFP.

Online retailer ASOS said that a $63 polka dot dress she wore in March sold out in "minutes".

A palace spokesman would not comment on how Kate chooses her clothes except to say that the Duchess does not have a style adviser -- something unusual for someone so prominently in the public eye.

But there is clearly a strategy behind her choices and even an element of "fashion diplomacy".

Kate wore a lacy dress by Canadian-born designer Erdem Moralioglu on a trip to Canada, and a silk dress by Singapore-born designer Prabal Gurung for her trip to Singapore.

In contrast, interest in the style of Prince William is nowhere near as keen as in that of his wife and son.

"People occasionally comment: who makes his suits? Why is he wearing that dreadful pair of trainers again?" Kelley said, adding that she had no plans to set up WhatWilliamWore.com.

"That would be going too far."