British archaeologists dug up the tiny coffin in Giza, Egypt, nearly 100 years ago, and it has sat in a Cambridge museum ever since.
For decades, researchers thought the small bundle inside was nothing more than a bunch of mummified internal organs, the sort of gruesome thing you end up with after a routine adult embalming.
But new CT scans show the remains are actually that of a fetus, the Fitzwilliam Museum in Cambridge announced this week.
The fetus is the first "academically verified" Egyptian mummy found at 16 to 18 weeks of gestation, according to the museum.
Julie Dawson, head of conservation at the museum, called the mummy an "extraordinary archaeological find that has provided us with striking evidence of how an unborn child might be viewed in ancient Egyptian society".
"The care taken in the preparation of this burial clearly demonstrates the value placed on life even in the first weeks of its inception," Dawson said.
Researchers believe this fetus was likely to have been the result of a miscarriage.
Other mummified ancient Egyptian fetuses have been discovered, but they have been older. Two mummified fetuses, probably 25 to 37 weeks old, were found in individual coffins within King Tut's tomb.
This newly discovered mummy probably dates to between 664 and 525 BC, according to the Fitzwilliam Museum.
The package inside the deteriorated 43cm-long cedar coffin had been carefully wrapped and bound with bandages and then covered in molten black resin.
Initially, efforts to x-ray the contents of the coffin yielded inconclusive results. But during preparations for a forthcoming exhibition on ancient Egyptian practices regarding death, the museum decided to have another look.
The coffin was re-examined at Cambridge University's Department of Zoology - this time with micro CT scanning.
"The cross-sectional images this produced gave the first pictures of the remains of a tiny human body held within the wrappings, which remain undisturbed," the museum said.
Scans clearly showed five fingers on both hands, five toes on both feet and long arm and leg bones. The fetus had its arms crossed over its chest, which, "coupled with the intricacy of the tiny coffin and its decoration, are clear indications of the importance and time given to this burial in Egyptian society", the museum statement said.