Standing in her Upper East Side condo, 28-year-old Karen is hoisting up a baby boy, revealing biceps that could give Michelle Obama a run for her money. The glamorous blonde whips out her homemade vegan apple walnut puree — fine for her 1-year-old, she says — but too indulgent for her. Karen went from an extra-small to a medium during her pregnancy, gaining 43 pounds, which left the former legal powerhouse feeling like, well, a house.

“I was obsessed with the fact that I didn’t become super-skinny right away,” says Karen, who asked that her real name not be used. “Every day, I strolled with my son and saw skinny moms around me — women who looked perfect, and I put crazy expectations on myself.”

That was true even before her son was born. Through most of her pregnancy, she continued her workouts at the gym, including weekly personal training sessions. By the end of her pregnancy, Karen’s abs were so strong, her gynecologist struggled to slice through her core muscles during the C-section.

“A year later, I’m back to my original weight, but I’ve accepted that my stomach will never look the same again,” she says, pointing to a barely perceptible “pregnancy pouch.”

“All of my relatives gush about how good I look — but here on the Upper East Side, I feel like every-one around me looks much better.”

Karen’s paranoia isn’t so crazy when you consider celebrities like reality TV star Bethenny Frankel, who lost 30 pounds within a month of delivering a baby girl last year. Rachel Zoe barely produced a bump before giving birth to a boy in March, while Ivanka Trump posed in a Playboy bunny outfit for Harper’s Bazaar last month. (Trump is due in July.) In the meantime, Victoria Beckham — due to give birth July 4 — is sporting a belly less pronounced than your average beer gut.

“Beckham’s in her eighth month now, and looking too thin continues to be part of [her] plan — healthy or not,” says Lisa Cohn, a registered dietitian in Manhattan. “She hasn’t been looking as vibrant — pale face, dull eyes and more drained. If [Beckham’s] extreme post-baby weight-loss plan of shedding 20 pounds in a short period is true, it sends a dangerous message to other women out there.”

Indeed, the A-list trend of gaining a scant amount of weight during pregnancy has led to a similar psychosis among pregnant alpha women in NYC. Call them the mommyrexics — a breed of new moms who are pressuring themselves to bounce back to fighting weight days after they’ve left the hospital. Squeezing into maternity Spanx after having a baby isn’t good enough. These moms want to starve and jog themselves skinny if it kills them.

“I’ve seen a massive uptick in my clients feeling stress if they’re not down to their prebaby weight,” says Rosie Pope, the “pregnancy concierge” and star of Bravo’s hit show “Pregnant in Heels.”

Pope, who owns her own maternity boutique on the Upper East Side, even had to add an extra-small size to her range this year, due to popular demand.

“I’m worried about this trend, because even I couldn’t fit into it, and I’m small!” says Pope, who has a 4-month-old son.

The worlds of fitness and maternity are now intertwined for figure-obsessed moms-to-be. Runningskirts.com is just one of several lines — including maternityworkoutclothes.net and maternityactivewear.com — boasting about its new “maternity collections,” which meet the needs of pregnant women who exercise, sometimes excessively.

So who’s most likely to fall victim to mommyrexia?

“Women whose beauty is part of their allure, women who are frightened about losing their husband’s attention. You do see women — and it’s a vicious cycle — who are worried about losing their husbands, and will ignore the baby and even come to resent the baby,” says Isaac Herschkopf, a Manhattan psychiatrist who treats celebs with eating disorders. Herschkopf says he has seen mommyrexia trickle down from Hollywood to the masses, especially in wealthier communities.

While the normal amount of weight a woman gains during pregnancy depends on her body type, Herschkopf stresses that more than other factors, when it comes to taking it all off, “You want to lose it consistently.”

Nonsense, say the mommyrexics. While they struggle to avoid gaining much weight during pregnancy, after the baby is born, they want to whittle back down to their fighting weight — within weeks. “I’ve heard of women who have opted to skip breastfeeding altogether so they can keep their day schedules open immediately for spin class, hot yoga and five-mile jogs through the park,” says 27-year-old Bryce Gruber, founder of the lifestyle Web site The Luxury Spot.

“I actually knew of a mom that was skipping meals and drinking a ton of diet sodas for energy without calories after her baby was born,” says Gruber.

Meanwhile, at fitness centers, the increasing number of programs purporting to help new moms stay slim reflect the growing trend.

Joshua Margolis is a certified personal trainer at Mind Over Matter Health & Fitness, a personal training service in Manhattan that specializes in mommy-focused classes — with plenty of MILF- and Stroller Boot Camp options.

“Hell hath no fury like a post-partum woman trying to get back into the groove,” Margolis admits.

“We try to ride that wave and contribute to it for as long as possible.”