Women are posting these photos to prove that their waists can hide behind A4 paper. They hold the paper vertically - where it is only 8.3 inches (21 cm) across - to completely cover their waists.

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According to Tech Insider, a waist that can fit behind this paper is 25 inches (63.5 cm) or less in circumference. To put that number in perspective, the average waist size of American women is about 37.8 inches (96 cm), according to USA Today.

This isn't the first body image selfie trend to hit china. Last year there was the belly button challenge, where women would post pictures of themselves reaching behind their backs and touching their navels.

The belly button challenge spawned a similar trend involving pictures of women grabbing their breasts from the same behind-the-back position.

According to Popsugar, there are already millions of photos with the caption #a4waist #challenge, and the phenomenon is spreading to other social media sites such as Instagram and twitter. Many celebrities are participating.

But not everyone is going along with this most recent fad, as there has been plenty of negative reaction.

Xiao Meili, a women's rights activist, called the A4 waist craze "utterly boring," going on to criticize the notion that "beauty can be measured."

Feminist Zheng Churan posted a picture of herself holding A4 paper horizontally at her waist. She wrote on the paper, "bodies don't need eyes staring at them. I love my fat waist."

Others have resorted to humor. Weibo user Joe Wong posted a photo of with paper covering his face, saying "I have A4 face."