Marilyn Monroe was once the personification of beauty with her cleavage, curves and blond hair. But for women today, that look is passé. Instead, a very different appearance is trendy, namely the “ab crack”. Never heard of it? That's because the ab crack, or the linea alba as it's scientifically known, is a tendon that's only visible in very well-defined abdominal muscles, if at all. More importantly, it's genetically determined, making it unattainable for most people.

Ever since model Emily Ratajkowski published a picture of her ab crack on Instagram, many girls and women have followed suit, posting photographs of their abdomens. It's not the first bizarre beauty trend that has taken off on Instagram.

The ab crack follows the “bikini bridge” — the bridge that's formed just above the belly when a person lies flat on their back and their bikini bottom touches their hip bones. Then there's the “thigh gap” — the space between the inner thighs when a person stands upright with their knees touching. There's also the “collarbone challenge,” which involves young women trying to balance as many coins as possible on their clavicle. The thinner the person, the more prominent the clavicle, the more coins fit on it.

What do these trends have in common? They all hold up extremely thin bodies as a beauty ideal. Being healthy — eating well-balanced meals and exercising regularly — doesn't give a woman an ab crack or thigh gap.

"These trends require extreme sport and a drastic reduction of calorie intake. You almost have to starve yourself,” says Ingo Froböse, a sports scientist. To make the ab crack visible, you should not have more than 12% body fat. Slender women have 18% to 20% of fat in their bodies.

The thigh gap trend, popularized on Instagram— Photo: thighgap_x via Instagram

Froböse says that the ab crack is a sign that a person is underweight. And getting it might still be impossible for most people as it requires certain genetic code. Some people will never have that dent in their body, no matter what they do.

In order to get the thigh gap, your legs have to be as thin as possible. This is extremely challenging because women naturally store fat in their thighs.

"In order to get the thigh gap, the fat reserves need to be burnt and muscles built slightly. Something that is often seen with extreme runners," says Froböse.

Just like the ab crack, genetics determine if a person can get a thigh gap. So even with an extreme diet and lots of sports, some women would not able to achieve this desired gap. “Depending on the direction of your hip bones, even women of normal weight can have a thigh gap. Knock knees on the other hand make it almost impossible, no matter how much you work out."

Out of all the extreme body trends, Froböse considers the bikini bridge to be the least extreme. While the abdomen needs to be defined and belly fat needs to be reduced, the bridge is “feasible with a healthy diet and workout routine,” he says.

That's definitely not the case with the “collarbone challenge.” Froböse says that the trend is the most dangerous of the lot. “Prominent clavicles are a characteristic of anorexia. Only extremely thin people can get there because you can't train bones," he says.

Psychologist Andreas Schnebel, who treats people with eating disorders, watches the Instagram trends with concern. "What's particularly worrying about it is that due to social networks, these pictures reach girls all over the world, who then try to imitate them. But for most of them, these ideals are unattainable."

Schnebel says that trying to achieve these body goals leads to bulimia and anorexia. “Anorexia usually also comes with mental problems or traumas…but the combination of an extreme diet and an excessive workout routine often triggers the eating disorder,” he says.

Schnebel increasingly sees a connection between Instagram body trends and illness. Whereas in the past, girls suffering from anorexia tried to hide their disorder by wearing loose clothing, now there's almost an exhibitionistic celebration of their illness, he says.

Perhaps what's more disturbing is that the disorder, and not the beauty ideal, is being sought after.

The "ab crack" is the newest body trend to spark social media outrage. Models including Bella Hadid, Emily Ratajkowski and Jourdan Dunn are in on the trend — Photo: emrata via Instagram

The so-called “pro-ana” movement involves a network of people suffering from anorexia and bulimia who celebrate their illness in public and encourage others to follow suit.

While Instagram body trends and the pro-ana phenomenon are not directly linked, “it most certainly does have an effect on the development of eating disorders, if pictures like these are constantly in your face," says Schnebel.

He could be right. A recent study found a direct link between television show "Germany's Next Topmodel" and the number of eating disorders in viewers.

The internet further perpetuates these trends. “Only 5 to 10% of girls in the real world actually look like their Instagram ideal," Schnebel says. “It makes girls feel ugly because they know it's impossible for them to reach their ideal shape with their physical shape.”

Although apps like Instagram ban certain hashtags to contain dangerous body trends, stubborn users find ways around them by spelling words differently. Bulimia becomes bulima. Thin is written as thynn.

What can parents do to address this problem? Schnebel does not advise parents to stop their children surfing the internet. Instead, he says, it's much more important to make children understand that physical appearance isn't everything.

"In therapy, we repeatedly see girls whose mothers are constantly on a diet and unhappy with their bodies," Schnebel says. He tells parents to talk to their children and explain to them that, "it's okay if you're watching that, but please also be aware of how ridiculous it is."