Two 3,000-year-old mummies discovered on the banks of the Nile have 'lower back' tattoos similar to those seen today.
The mummies were found about a century apart in Deir el-Medina in Egypt, but the tattoos went unnoticed as they are barely visible in normal light and were only revealed under infrared.
One mummy found approximately 100 years ago has a band of diamonds across her back, while the other, discovered in 2019, has a water pattern and symbols of plants.
Scientists said the tattoos could be linked to pregnancy and menstruation.
The US researchers wrote: "Acute lower back pain frequently accompanies labour, and, in some women's cases, menstruation, as well.
"However, it is important to consider ubiquity in this interpretation.
"If the tattoos were intended to ensure successful pregnancy and childbirth, or avoid pain for the woman who bears them, we would expect to find the presence of these markings far more frequently."
The tattoos had gone unnoticed as they were obscured by papyrus and darkened, embalmed skin.
Infrared technology changed that, said researcher Anne Austin, assistant professor of history at the University of Missouri - St Louis.
The human eye can only see a certain range of wavelengths, but infrared photographs allowed Austin to see beyond that range, revealing the tattoos.
Austin said in a previous interview: "The more people read about our new research on tattooing, the more I hope they reconsider the ways we view people with tattoos and the long history of tattooing across cultures."
While the identities of the mummies are unknown, it is understood that artisans who crafted royal tombs worked at Deir el-Medina.
All seven bodies Austin examined were female, and she has said that their tattoos likely indicate roles as healers or priestesses, rather than artisans.
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