There’s no question maternity fashion is making headlines on both sides of the Atlantic. With fashion darlings the Duchess of Cambridge, Fergie, Kim Kardashian and Margherita Missoni all becoming pregnant at about the same time, how to dress your bump has become the most popular style subject of the moment, with the mothers-to-be rising to the occasion.

On her first visibly pregnant outing, in February, the Duchess, whose baby is due in July, wore a Max Mara grey wrap dress. More recently she has taken to covering up in coats, like the pale green tweed Mulberry number worn for an engagement at Windsor Castle. Kardashian, meanwhile, has been seen in Valentino, Salvatore Ferragamo, Lanvin and Saint Laurent, while Fergie sported a black tux to an event in Brazil, followed by a clinging rhinestone-bedecked mini with a long train earlier this month. Missoni has been wearing, well, Missoni.

All are following the lead of former French First Lady Carla Bruni-Sarkozy, who wore a Chanel haute couture white tweed dress and black jacket while pregnant, in adapting non-maternity wear to their new situation.

Indeed, this has become such a trend that you’d think it might give the fashion world ideas. According to the maternity wear line Séraphine, in the UK there are about 800,000 births a year and, on average, women spend £130 on maternity clothes (a total of £108m), while in the US, with just under 4m births a year, spending is around $800m (an average spend of $200). And yet maternity dressing is the last great unexplored frontier in style. High fashion brands have gone into children’s clothes, wedding gowns, lingerie, sportswear, even linens. Many pregnant women clearly don’t want to surrender style any more but, when the topic is raised, catwalk stalwarts respond with an awkward silence.

Diane von Furstenberg (whose wrap dress many would argue is perfect for maternity), Jason Wu and Temperley London all politely declined to take part in this piece, for example. “It is not a story that we feel is the right fit for us. We don’t do any maternity wear, and our relationship with the Duchess of Cambridge is an important one we never comment on,” said Temperley London.

Valerie Steele, director of the Museum at the Fashion Institute of Technology in New York, believes pregnancy has an image problem. It lacks “the glamour and sleekness of the ideal female figure. Pregnancy involves a profound change in the female silhouette, and a change which superficially resembles obesity. That is something that is very difficult for designers.”

Steele recalls how during her own pregnancy, while searching for something stylish, she was mulling over Issey Miyake’s Plantation line, in particular a loose, cotton dress that was breezy and classic 1980s, when “someone in the store looked at me and asked, ‘Is it twins?’” – the implication being Steele looked fat. “I ran out of the store crying,” she says. Still, she adds, “It seems like a missed opportunity” for designers.

Susan Lazar, a New York designer for 20 years, who now creates the popular Egg Baby line and does occasional maternity wear (simple jersey dresses and accessories) says she believes designers avoid maternity because it has such a short lifespan. “No woman wants to wear their maternity clothes after the baby arrives,” she says. “You want to burn them.”

There’s also the practical issue of patterns. “Maternity patterns are different from regular clothes because the belly picks up the front and, of course, you need larger waistbands,” Lazar says. She points out that launching a maternity line would require real investment, which might not be worth it as long T-shirts, leggings and layering remain popular as pregnancy wear. In fact, she says, many women can almost get away with buying entirely non-maternity basics until the last trimester. “It’s not really necessary to buy a maternity wardrobe, so designers don’t bother making one,” she says.

Los Angeles stylist Nicole Chavez worked with then-pregnant Kristen Bell for this year’s Golden Globes and put the actress in a loose-fitting (non-maternity) Jenny Packham dress for the event. As a rule, she emphasises non-maternity wear for her clients, including Jessica Simpson, whom Chavez has dressed throughout both her pregnancies partly in Dolce & Gabbana (it has a lot of stretch in the fabric, she notes).

“She is a curvy girl and quite petite, so it was important to show her bump,” says Chavez. Issa is also a favourite non-maternity line for maternity wear, because the signature V-neck is flattering on an ample bust and the pleating at the waist flattering to a baby bump.

Still, given the marketing boost the current crop of pregnant style-setters can bring, it would be good to see a brand respond. It could, after all, give birth to a whole new creative niche.