Adam Smith, 37, made the gruesome discovery after he was sorting through the possessions at Barbara Smith’s apartment following her death on July 21 aged 68.

He had moved in to look after his mother after her health started to deteriorate following her lung cancer diagnosis. While deciding what he was going to keep or throw out, he went to see what was inside a box which had been kept in his mother’s freezer for years.

“I pulled it out and I set it directly right here [kitchen table] I grabbed this knife and then just cut through the plastic and opened it up,” he told KSDK. “It turns out it was a baby.

“It still had skin, hair and everything. It was mummified. After that, I just freaked out I put it back into the box. And I called police right away.”

Adam Smith said for years he assumed that the box had contained a piece of wedding cake or even money as his mother did not have a bank account. His mother even took the box with them when they moved homes.

His mother had been living in the Clifton Heights apartment where she died more than 20 years ago.

Adam Smith said he had never thought to look in the box in the freezer before.

“I’ve asked her several times. It was either a ’no no’ conversation [or] she blew me off. My mom has always been so secretive about things throughout life.”

Speaking to the St. Louis Post-Dispatch, Adam Smith said he believes the dead infant was a sister of his. He said he has since recalled previous conversations he had with his mother when she appeared upset, which could help shine a light on the baby’s identity.

“All I can remember is that she told me, ‘My oldest child would have been 21 today,’” he said. “And that her name was ‘Jennifer.’”

Adam Smith added that it is possible that that his mother could have kept the baby a secret for nearly 50 years until it was discovered.

“We’re saying what 46, 47 [years]? That’s how long, I’m assuming it is my sister, has been in a box in the freezer.”

The 37-year-old said that his mother never divulged any secrets about herself even while she was on her deathbed.

“That’s what makes me think maybe she did something to this baby and didn’t want to tell anyone, because she was afraid she would get in trouble,” he told the St. Louis Post-Dispatch.

Other family members have since said that Barbara Smith had also previously given birth to twins: one being a stillborn and the other given up for adoption.

“I’m finding out my mom wasn’t who I thought she was,” Adam Smith said.

Police said they have launched an investigation into the discovery.