A St. Louis man cleaning out his dead mom’s freezer assumed he was opening up a decades-old wedding cake — but it turned out to be a dead baby, according to a report.

Adam Smith, 37, now believes the frozen corpse he found this week wrapped in a pink fleece is his older sister, who passed away years before he was born, according to KSDK.

“We’re saying 46 or 47 [years] — that’s how long. I’m assuming it is my sister, been in a box in a freezer for this long,” Smith told the network.

“I’m 37 … and I was always told it was a wedding cake top. It turns out it was a baby.”

Smith said the well-preserved cadaver still had skin and hair.

He recently moved in with his mom to care for her until her death last week after a battle with cancer.

Throughout his life, whenever Smith mentioned the mystery box to his mom, she brushed it aside.

“I’ve asked her several times, it was either a no-no conversation or [she] blew me off. My mom has always been secretive about things [in] life,” he said.

“I’m more confused, angry. I just want to find closure, I want to find more answers.”

Police said they are still investigating the discovery.