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".