On a recent Friday, the bathing-suit department at Bloomingdale’s on 59th Street was bustling. Pat Walsh, 54, a slim blonde administrative assistant from Staten Island, was inspecting a retro-inspired red polka-dot Anne Cole one-piece with thick straps and a bosom-enhancing sweetheart neckline for an impending trip to Punta Cana, in the Dominican Republic. “I always think more is better,” she said. “Like, more of a swimsuit, more coverage.”

“Sometimes they all look the same to me, these suits,” she continued. “But I just thought, ‘Wow, I haven’t seen a polka-dot suit—I don’t know if I’ve ever seen one! So I’m going to try it on. I think it would be flattering to me.”

This season, swimsuit designers have pillaged the demure 1940s and 1950s and produced a barrage of modest, coy options like the polka-dot Anne Cole one-piece admired by Ms. Walsh. These are suits that channel Marilyn Monroe circa 1945, back when the leg was titillating enough and the thong was but a twinkle in some evil man’s eye. When famous women had curves that swimsuits cinched and smoothed and accentuated. When swimsuits helped things, in other words, rather than stranding a girl alone with her stomach flab like their successor, the string bikini.

On the high end are Norma Kamali’s much-coveted Bill Mio suits ($350), which have the ruched bodice and generous proportions of the suits ’40s swimming champion-turned-sexy movie star Esther Williams wore. (Interestingly, Ms. Kamali started making swimwear in 1970, at the advent of a long, dark American love affair with skimpiness. This culminated in the ’90s with the Brazilian-exported thong, at which point women said, collectively, Please!)

On the low end, trendy, youthful retailers like Urban Outfitters and Juicy Couture have embraced modest, sweetheart-necklined one-pieces, perhaps meant to be worn ironically to one’s Hamptons share house or to sunbathe at McCarren Park. After all, for the fit 20-something, a gratuitously modest suit is almost more transgressive than an eentsy bikini. It says, approximately: My body is so hot that I don’t even have to show you how hot it is.

‘A Little Tension’

“I like how the ruffles cover your chest and add a little tension without revealing too much,” said designer Araks, who makes a popular retro-inspired two-piece with a halter top and high-waisted bottom that retails at La Petite Coquette. “It’s flirty, but it still has an innocence to it.”

At Macy’s, reissued Jantzen one-pieces, marketed by a glossy ad campaign featuring the model Carolyn Murphy (whose sex tape disappeared quickly, demurely from the collective radar), have been flying off the shelves this year and hard to find online since March in sizes under 8. “This whole retro trend is in general in the fashion world,” said Nicole Fischelis, the store’s fashion director. “There is a fusion of contemporary, today and yesterday, whether the ’40s, ’50s or ’60s.”

But at Bloomingdale’s, the retro trend—rather than striking customers as a welcome reprieve from the low-slung bikini or a subversive fashion statement—seemed to be perplexing more than a few (retro styles by Anne Cole, Carmen Marc Valvo and Juicy Couture abounded, but the string and bandeau bikini still ruled the sales floor). “Probably for someone who’s not particularly athletic or sporty,” sniffed Kathy Voss, 30, a tourist from London. “It’s a more ‘look at me’ swimming costume, rather than the ‘I’m going to swim 20 kilometers.’”

Then again, this is precisely the costume required by the lazy-skinny girl of the moment, the one who eschews Equinox for Spanx, just like grandma.

But “if you’re in your 50s or 60s, that kind of look isn’t going to work,” argued one shopper, Patricia Keenan, 55, an artist. “It’s more va-voom, a younger look. Otherwise, it looks kind of silly.”

Indeed: this season’s retro swim trend began with aggressively undemocratic high-fashion runways, not with an altruistic desire to make swimsuit shopping more pleasant for the average middle-aged beachgoer with trouble spots. Designers like Stella McCartney, Michael Kors and Miuccia Prada sent full-coverage swim bottoms with retro details like halter tops, belts and ruffles down the runway between 2004 and 2007, accessorized with turbans, oversize sunglasses, diaphanous shirts and high, high heels, and suddenly, swimwear was glamorous again. (Fitness trends, beach volleyball, and US Weekly’s continuous stream of emaciated, bikini-clad celebs hadn’t been helping things.)

A barrage of magazine spreads followed, including, memorably, Scarlett Johansson’s April 2007 Vogue cover, which featured the starlet in vintage-inspired Prada and Dolce & Gabbana suits accessorized with red lips and high heels.

“Every year you see more swimwear on the runway,” said Ms. Fischelis.

“We started seeing [retro suits] on magazine covers probably two years ago, and it became very aspirational,” said Donna Wolff, vice president and divisional merchandise manager of swim at Bloomingdale’s.

From there the trend gained momentum: ’40s-issue haltered or strapless ruched one-pieces and high-waisted, halter-topped two-pieces have continued to inspire glossy magazine photo shoots this summer, and have also saturated the market in a downward direction, popping up at J.Crew, H&M and, most notably and extensively, Urban Outfitters, home to an improbably large and well-trafficked online bathing-suit shop that currently offers full-coverage one-pieces in gingham and plaid.

Customers are “going for a total look,” said Ms. Wolff. “They’re going to the beach and, instead of wearing a T-shirt and shorts, they’re wearing a cover-up.”

In other words, this is about fashion, not fat. Swimsuits, after all, popped up in the early 20th century as utilitarian objects meant to allow women the freedom of movement to swim. Frenchman Louis Reard turned them into sex objects with his invention of the bikini in 1946. Have women, in effect, taken them back by turning them into conscious fashion statements—i.e., refocusing the attention on the suit, not the boobs?

Ms. Wolff was unconvinced. “If you ever put on one of those suits, it’s a very tight-fitting suit,” she said. “It’s a sexy swimsuit. The retro thing is not all about being covered up.”

Flesh and Fureblows

However tight, some women—rather than heralding retro swimsuits as instruments of liberation, allowing us make our beach-side look more about style than how many crunches we’ve done lately—would prefer to look overtly sexy than fashionable, thank you.

“I just feel more feminine in a bikini,” said Denise Margulies, 41, a personal trainer who lives on the Upper East Side, back at Bloomingdale’s. “I feel sexy in a bikini. I don’t even want to tan in a swimsuit like that”—she motioned dismissively toward a rack of colorful solid-colored Carmen Marc Valvo one-pieces—“and I love tanning. I like the belly button to show. And I’m a mother. I feel too matronly in a suit like that.” (Though, it should be noted, Kate Winslet’s character in the 2006 movie Little Children selected a red retro-inspired, plunging-neckline one-piece for her coming-out as a MILF.)

The retro suit, Ms. Margulies speculated, is probably best left to “very busted, very buxom women. Not athletic-looking bodies. Very full-figured. Not that there’s anything wrong with full-figured.”

Whether targeted at lithe hipsters or women whose bodies channel the original pin-ups, who were much fleshier than today’s most glamorous stars, the retro trend is not undergoing its first revival. “Silhouettes and details that were popular in the 1940s and 1950s are back,” proclaimed The New York Times in 1990. “These include wide shoulder straps, plunging halters, midriff shirring, front draping, attached sheaths and skating-style skirts.” (O.K., fine, maybe we can leave the “skating-style skirts” in the last century.) And now, cheap, ubiquitous mass brands have lent the whole enterprise of shopping a “Why not try it once?” feeling, ushering in a modern era when trends move faster than ever, trend loyalty is nonexistent and a woman might buy a modest, retro Jantzen one-piece a few clicks after purchasing a barely-there Melissa Odabash string bikini.

But if swimwear is now subject to fashion’s breakneck silhouette changes and also its wider range of choices, there is also perhaps a lost, essential glamour that has propelled the rise in modern interpretations of the pinup swimsuit, specifically: pre-Britney, pre-celebrity-cootch-flash, pre-“Reese: Pregnancy or Bloat?” headlines, harking back to a time when flesh still had the dint of the new, when sex was something to suggest rather than to have on YouTube, when we weren’t all exhausted and financially depleted by our draconian waxing and lasering routines. “It’s much more about sensuality than sexuality,” said Ms. Fischelis.

Recalled Gloria, a saleswoman at Bloomingdale’s, with no small amount of nostalgia: “Last year, we had a white suit by Carmen Marc Valvo. A young woman in her 20s, she bought it. Oh my God, she looked like a throwback from the ’50s. The ’40s or ’50s. She’s tall, I think she was blond. … And it was a creamy suit, and she looked—oh! She really looked good.”

Additional reporting by Leigh Kamping-Carder