The floral blue wrap dress by British maternity brand Seraphine, described on the website as a "chic short-sleeved maternity dress", is available in two other colours but both have sold out and the blue version now boasts a month-long wait.
This follows the news that Jenny Packham's online store crashed due to enormous traffic after fans found out that the Duchess of Cambridge was wearing one of her designs to leave the hospital. Prince George's first baby shawl has also sold out online.
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HELLO! Online spoke to the founder of Seraphine, Cecile Reinaud, last week.
"It's wonderful to see Kate in our signature floral dress just a day after giving birth ," she told us.
"This dress is coincidentally called Blossom and it's a perfect choice for a new mother who is clearly overjoyed by the birth of her first born."
Kate is a well-known fan of the maternity brand, and Cecile told us what effect this had had on Seraphine.
"The Kate effect is huge and we have been enjoying a 40% growth in sales since it became public that Kate bought Seraphine dresses back in April 2013," she said.
"A few weeks ago she was photographed wearing our Renata Spotty dress whilst shopping on King's Road and our sales boomed immediately with the dress completely selling out.
"Subsequently, our customers have put themselves on a four week waiting list to get hold of one!"
Speaking about Kate's maternity style, Cecile added, "Kate has managed to stay true to her feminine and chic style during pregnancy . She has loved a bit of polka dot and soft prints and has continued to wow us with her stunning figure and legs by opting for slightly shorter hemline and accentuating her empire line.
"She has shown the world that being pregnant is a fabulous opportunity to look radiant and confident about your new curves."
Asked about the soon-to-be-iconic moment when Kate stepped out of the the Lindo Wing with her newborn baby in her arms, Cecile said, "Kate looked absolutely radiant. Her make-up and hair were very natural and yet made her look stunning.
"It was very emotional to see her almost teary with emotion with presenting her newborn to the world. I always thought she would have a boy despite the rumours because there is a popular saying that "girls steal the beauty of their mum during pregnancy" and in her case she looked so radiant all along I suspected she had a boy!"