Soon after completing a nine-mile run,Trish Staine started having excruciating pains in her back and put it down to her gruelling two-hour training session.
But a day later, she was cradling her newborn baby daughter!
Trish, 33, said she had no idea she was pregnant before her surprise birth.
The mother of three, from Duluth, Minnesota, said she hadn't gained any weight or felt any movement in the months before. Added to that, her husband had had a vasectomy.
"I said 'no, no, that's impossible'," she said. "I definitely thought I was done having kids."
Trish and her husband, John, have a daughter, seven, and a son, 11. She's also stepmother to John's three boys, ages 17, 19 and 20. She said she ran for about two hours on Sunday in preparation for the Garry Bjorklund half-marathon on June 22.
"I had a sore back Sunday evening. I had taken a hot shower and was dealing with it," she said.
"Monday morning, I woke up and had more back pain, and as the day went on it got worse. I thought I should go to the ER. I thought I ruptured a disc or pulled a muscle."
But she soldiered on, watching her husband play basketball at noon and going to her daughter's short play.
When Trish got home, she thought a bath might help her pain. As she talked to her husband on the phone, Mrs Staine said her pain was becoming unbearable. Her husband called an ambulance.
"I felt like I was dying. I didn't know what was going on," she said.
During the examination, Trish was stunned to learn medical staff had detected a foetal heartbeat.
She was whisked to the delivery room and in what she said seemed like five minutes later, her daughter - who the couple have named Mira, short for Miracle - was born five weeks early at 3:25pm on Monday, weighing 6lb 6oz. The couple hope to take their baby home in about a week.
Marathon Runner Mum Thought Surprise Baby Was Backache!