DULUTH, Minn. (AP) — An aspiring half-marathon runner in Minnesota attributed her unbearable back pain to a two-hour training session. A day later, she was cradling a newborn.

Trish Staine, 33, says she had no idea she was pregnant before Monday's surprise birth. The Duluth mother of three said she hadn't gained any weight or felt fetal movement in the months before. And besides, her husband had a vasectomy.

"I said 'no, no, that's impossible,' " Staine said Wednesday from her Duluth hospital room.

"I definitely thought I was done having kids," she joked. Staine and her husband, John, have a daughter, 7, and a son, 11. She's also stepmother to John's three boys, ages 17, 19 and 20.

Staine said she ran for about two hours 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," Staine 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 Staine got home, she thought a bath might help her pain.

As she talked to her husband on the phone, 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 emergency room examination, Staine and her husband were stunned to learn medical staff had detected a fetal heartbeat. She was whisked to the delivery room and in what she said seemed like 5 minutes later, her daughter was born at 3:25 p.m. Monday. She weighed 6 pounds, 6 ounces, and was 18.9 inches long.

Staine said her husband has a good sense of humor.

"He's still in shock. Everybody is teasing him," she said.

Born about 5 weeks early, the Staines expect they will be able to take their baby home in about a week, a girl they have named Mira — short for Miracle.