An ultrasound on an Israeli mother-to-be earlier this year showed her baby had an enlarged stomach, but nothing could prepare the mother or her doctors for what they found after she was born.
Inside her was the embryo of her 'twin' - complete with bones and a heart.
"When the woman came to give birth, we got the impression that the baby had a big tummy,"Omer Globus, director of neonatology at Assuta Medical Center in Ashdod, told the Jerusalem Post. "Because of that, the moment the baby was born, we checked her and realised that we can feel something inside her stomach, so we did ultrasound tests among others.
"We were surprised to discover that in her stomach was a partially developed fetus."
They immediately went to work to take it out.
"The impression we got is that there were originally two babies, and we're checking it," he said.
The surviving baby stayed in hospital for a few days, and was released.
It's not clear how it happened, but there are two main theories. One is that the mother was originally pregnant with twins, but very early on one got absorbed by the other, so couldn't develop into a baby.
"It happens as part of the fetal development process when there are cavities that close during development and one of the embryos enters such a space," Dr Globus told the Times of Israel. "The fetus inside partially develops but does not live and remains there."
The other is that there is only one baby, the failed embryo being a kind of tumour called a teratoma - containing all the cells needed to form organs and body parts.
When it happens in a newborn baby it's called 'fetus in fetu. The rare phenomenon happens about once in every 500,000 births, according to a 2010 study, but not all are discovered.
In 2017 a tiny skull containing a malformed brain was found in a Japanese teenager's ovary, and that same year saw a baby boy in India born with a fetus behind his stomach bearing arms, legs and a brain, but without a skull. In both those cases, surgery to remove the misplaced fetuses was successful.