San Francisco (CNN Business) Tumblr says it is banning images and videos that feature "adult content," including pornography, from its platform. The change, which will start December 17, threatens to alienate some of the blogging website's most active communities.

The company said Monday that it will no longer allow any images or videos of sex acts, nor will it allow "real-life human genitals or female-presenting nipples." It will make exceptions for non-sexual content, including breastfeeding photos and "health-related situations," such as sex-reassignment surgeries and mastectomies.

The decision comes weeks after Tumblr's mobile app was removed by Apple from the App Store. Soon after the app was removed, Tumblr acknowledged that it had found and removed material related to child sexual abuse on its website.

At the time, Tumblr said the material appeared on the website because it had slipped past a filter. The company scans images uploaded to the website against an industry database. The content in question had not yet been included in the database, according to Tumblr.

By expanding its list of banned material, Tumblr said it wants to make more people " feel comfortable expressing themselves " on the platform.

Tumblr will still allow written adult content, including erotica, fan fiction and other creative writing.

The company will enforce the new rules through a combination of automated detection, human moderation and assistance from community members who flag objectionable posts they find. Accounts found in violation of the rules will be made private, though the owners will be given a chance to appeal.

Tumblr, which was founded in 2007, quickly drew young users and became a haven for creative fan communities. It was sold in 2013 to Yahoo for $1.1 billion, a company that itself was acquired by Verizon last year. A year before the Verizon sale, Yahoo wrote down the value of its Tumblr purchase by $482 million, citing low projections for its performance.

An estimated 21.3 million people in the United States use the website at least once per month, according to eMarketer. A 2016 study by researchers in Italy found only 0.1% of users uploaded pornographic content to the website. But the same study found that 22% of Tumblr users liked or reblogged such material, while another 28% were unintentionally exposed to it.

The decision to ban adult images could have a major effect on some of the website's most active posters, including its fan communities. These groups often write stories and create graphic art based on TV shows, movies or books they love. Some of the work is sexual in nature.

"There's a lot of danger of false positives," said Casey Fiesler, an assistant professor at the University of Colorado Boulder who researches social media, regulation and fandoms. "I think that the fandom community in part has been really burned by this in the past, and they are rightfully freaking out right now."

It wouldn't be the first time strict content standards caused people to leave a service. Fiesler said that's what happened to LiveJournal 10 years ago. The popular blogging site started prohibiting what it considered objectionable content , and a large number of accounts were deleted. Users moved on to other platforms, including Tumblr.

Fiesler said fan communities could move elsewhere, including to new platforms such as Pillowfort. She has also researched the migration of many groups to more isolated but private communities, such as the chat app Discord.

Other groups could be hit by the change, too, including LGBTQ users, sexual assault survivors and sex workers who use the service to connect with each other and talk about safety issues, Fiesler added.

"I worry about this kind of thing having a disproportionate impact on people who use social media to connect with others who are struggling with issues of their own sexuality or gender identity," Fiesler. "This is a place they can also go to find support."

Even if the new rules drive away users, the issue might not matter much for Verizon, Tumblr's parent company. Tumblr is now bundled in a collection of media properties that was until recently called Oath. The division has since been rebranded as Verizon Media Group.

"It's safe to say that Tumblr is pretty immaterial to Oath these days -- there is relatively little ad revenue there," said Brian Wieser, an analyst at Pivotal Research who covered Yahoo before it was acquired by Verizon. "Changes probably wouldn't be noticeable to Oath, let alone to Verizon."