It's a dress specially cut for easy nursing access, according to the Britain-based maternity clothier.
And Kate wants more!
"She's ordered a couple of breast-feeding styles from us," Séraphine owner Cecile Renaud tells E! News.
Today's style "in particular was in a print, a light purple, quite a regal color, and we know she likes her floral prints so it was very nice."
So though Kate's frock photographed on the blue side as she waved at the adoring throngs outside Kensington Palace from the backseat of the family's Range Rover, it was indeed lavender (if not quite royal purple).
And about a week ago, Renaud says, they got a call for more.
"We've been asked to supply our range for breast-feeding," reveals Renaud, who also notes that the demand probably won't just be coming from the royal family, considering the heat wave sweeping the nation.
"We'll probably have a couple more orders on that front, we believe," she says. "With the temperatures with which we're having in London, [new moms should] definitely dress with cotton, natural fabric—let your skin breathe. You do tend to be quite hot in this postnatal phase, so keeping with fluids and natural fabrics...allow your body to recover from the shock of delivering your baby."
Kate, whom we imagine will have all the time and amenities she needs to fully recover, gave birth to her son at 4:24 p.m. (British Summer Time) on Monday. The wee Prince of Cambridge met the world last night at 7:14 p.m. as his mum and dad carried him out of London's St. Mary's Hospital, and his full name—George Alexander Louis—was revealed today.