Scientists looking at the first pregnant Egyptian mummy think they figured out why none were ever found before

• Scientists who examined a pregnant mummy explained why they think no others had been found.
• It might have to do with a chemical reaction that dissolves the bones of unborn children.

Scientists working on mummified remains from Egypt recently made a huge discovery: a set of remains they thought was a man was actually a woman — and a woman who was pregnant.

Before the Warsaw Mummy Project analyzed the remains, no one had ever spotted a fetus in a mummified body before.

Wojciech Ejsmond, a Warsaw Mummy Project scientist who led the study, told Insider on Friday that this had always seemed weird.

"Women in reproductive age were maybe not constantly pregnant, but every few years they would have been pregnant," he said. So why was there no proof of pregnant women who died being mummified?

Fetal skeletons — the usual way to spot a developing baby in this kind of case — never appeared on X-ray scans. It took the scientists developing a technology that wasn't looking for bones.

"Radiologists were looking for bones, and our case shows that, actually, you shouldn't. You should look for the soft tissue with a unique shape," he said

In a letter published on December 30 in the peer-reviewed Journal of Archaeological Science, the researchers gave a hypothesis for why the fetus might have disappeared from scans. It essentially comes down to pickling.

"It's like an experiment with an egg. You put an egg with an acid, the eggshell is dissolving, leaving only the inside of the egg," Ejsmond said.

"When the acid is evaporated, you have a pot with just an egg covered in minerals," he said.

Something similar probably happened in the mummy's body.

As the body decomposes, it starts naturally acidifying. "Formic acid is appearing in the blood, which makes the environment in the body more acid," Ejsmond said.

When that acidic environment hit the dead fetus, the bones almost all dissolved. The remnants of the chemical reaction, a bunch of minerals, were scattered in the water that was left in the uterus, the scientists hypothesized.

This made the little body virtually invisible to X-ray scanners.

"One way to explain it is that it pickled it, basically," Ejsmond said.

So why then did the mother's bones not dissolve?

That is because, during the mummification, the body is covered with natron salt to dry out the body. The act of drying it out captured the minerals in place, Ejsmond said, so the bones could still be spotted.

So far, the mummy studied by the Warsaw Mummy Project is the only one to be believed to have been mummified pregnant.

"But further research may show that it's more common than we think," Ejsmond said.