The fetus was discovered during a scan by researchers led by the University of Warsaw in April 2021. The finding suggests that there may be other mummies in museums that are currently "hiding" unidentified fetuses.

An embryo found in the belly of an Egyptian mummy last year was preserved for more than 2,000 years due to the acidification (lowering of the pH) of the woman's body as it disintegrated. Without oxygen entering the body, the chemical reactions that take place produce acidic compounds, such as formic acid. "The pH of the blood in the corpses, including the contents of the uterus, drops significantly, it becomes more acidic," the researchers wrote.

This is the conclusion of a team of researchers from the University of Warsaw, who discovered the presence of the remains of the unborn child using a combination of computed tomography and X-ray in April last year.

The mummy of the Mysterious Lady. [Credit: Warsaw Mummy Project]

Researchers said that the acidification process is more serious in mummies because sodium, a natural salt that accumulates in and around the body during the mummification process, creates a barrier that traps acid in certain places, such as the uterus. "The end result is an almost hermetically sealed uterus that contains the fetus," the researchers wrote.

The acidic conditions inside the mummy's uterus would not be strong enough to disintegrate the fully formed human bones. However, they could have disintegrated the fetal bones because "the [bone] metallization is very weak during the first two trimesters of pregnancy and accelerates later," the researchers wrote. However, the remaining soft tissues that make up the fetus would have remained largely intact.

The fetus remained in the untouched uterus. In the photo: two volumetric yields of the fetus from the CT data. The labels on the right represent the head (A), the right hand (B), the left hand (C), and the right foot (D). [Credit: Warsaw Mummy Project]

The mummy is believed to be the first embryonic specimen known to contain an embryo and was removed from Egypt by Jan Wężyk-Rudzki, who donated the specimen to the University of Warsaw in December 1826.

A great deal of uncertainty surrounds the adult mummy - dubbed the "Mysterious Lady" - with experts still unsure of who she was and what exactly caused her death at the age of 20 in the 1st century BC.

Even the location of her tomb has disappeared from history, with some records showing that it was found in the "Royal Tombs of Thebes" and others in the Pyramid of Giza.