You know, the tattoo with a heart/uterus-like shape that's usually placed where the womb should be.
https://dic.pixiv.net/a/%E6%B7%AB%E7%B4%8B
https://danbooru.donmai.us/wiki_pages/79104
It's everywhere nowadays whether it's hentai or not, but I don't think I've ever seen the "demonic pubic tattoo" thing in a hentai prior to the mid-10s. Back then demon girls are just demon girls with either full body tattoos or none but then these demonic crotch tattoos dominated the market before I realized it. What I want to know is when exactly did this become a trend, what are the earliest works sporting it, and which works are most responsible for spreading the trend across the entire pixiv (and by extension, the larger hentai) community?
The oldest images with that tag on pixiv are the full body tattoos kind. There's a few with only crotch tattoos, but they're either some esoteric lines or a very simple solid heart shape unlike the complicated designs we see today. I don't think I've found any uterus-heart design that are commonplace today before 2014.