As part of a visual rhetoric course at the University of Waterloo, the Toronto-based poet and artist Rupi Kaur has been working on a photo series with her sister Prabh about menstruation. The photos are understated, grainy, totally non-graphic; their purpose, Kaur says in an artist's statement, is to demystify and destigmatize the female body — to make viewers "realize these are just regular, normal processes," nothing to reject or shame or shun.

How fitting, then, that within 24 hours, Instagram took her photos down.

On Tuesday evening, Kaur received a message from Instagram that a photo of a woman lying in bed — fully clothed, with a blood stain on the sheets — had been removed for violating the site's "community guidelines." (Those guidelines only formally forbid nudity, illegal activity and images that glorify self-harm.) But when Kaur reposted the photo, Instagram removed it a second time — provoking Kaur to pen a sternly worded open letter to the site that's since been "liked" more than 54,000 times on Facebook.

"Thank you @instagram," Kaur wrote, "for providing me with the exact response my work was created to critique ... when your pages are filled with countless photos/accounts where women (so many who are underage) are objectified. Pornified. And treated less than human. Thank you."

The ensuing fury was so intense, on Instagram and in other corners of the Internet, that Instagram eventually restored Kaur's photos — even emailing her personally to apologize for the "error." But Kaur is, frankly, unimpressed. She and her supporters see it as part of a larger problem with censorship on the platform.

"They allow porn on Instagram, but not periods?" Kaur asked The Washington Post. "... How dare they tell me my clothed body, the way I wake up at least once every month, is 'violating' and 'unsafe?' "

This is, incidentally, a question that Instagram and its corporate parent, Facebook, face with exhausting regularity: Why did they take down that breastfeeding photo? Or that piece of classical art? Or (fill-in-the-blank controversial thing), usually a lady's body parts?

A mere two weeks ago, a French court agreed to hear a civil case brought by a man who thought that Facebook had violated his rights when it removed a nude painting from his page. That followed closely on the heels of a British drama, in which a mother claimed Facebook removed photos of her breastfeeding.

Facebook recently clarified its moderation policies to make it unavoidably clear that nudity in fine art, breastfeeding photos and cancer-recovery photos are allowed. And yet, mistakes like this one are still common, and the processes around enforcement are totally opaque. Just this Monday, the very day Kaur uploaded her Instagram, the Electronic Frontier Foundation called on Facebook not just to clarify but to fix its policies, which "fall short" and "have often left users confused."

Without a doubt, much of the inconsistency has to do with individual moderators: Facebook and Instagram both receive a mind-boggling number of content reports, which are screened by hundreds of people around the world. By all accounts, those moderators receive pretty detailed guidance on what content they should and should not remove. But mistakes happen; things fall through the cracks.

"A member of our team accidentally removed something you posted on Instagram," the company's email to Kaur said.

Kaur, however, is not entirely convinced: "I truly don't believe it was a mistake," she said. "A mistake once maybe, but twice?"

Meanwhile, she points out, Instagram has a long history of censoring photos that it deems "too natural" — rather like Western society, at large. In fact, Kaur's three-day battle with Instagram evokes an argument she made in her initial academic proposal for the menstruation photo project.

"I was made to feel (a)shamed of my period before I even got it, and once I got it, society began to enforce its own shame on me," Kaur wrote. "Why is society so afraid of it? Why is the period so taboo? ... We highlight the sexuality of women in photographs but (we do not photograph) this."