A Duluth woman training for a half-marathon thought she had pulled a muscle or ruptured a disc in her back before she went to a hospital emergency room Monday.

When doctors told her that her pain was actually due to an impending birth, Trish Staine said, she was in denial.

Staine, 33, said she told doctors a baby was impossible. Her husband, John, 47, had had a vasectomy, and there were no signs of pregnancy — no weight gain and no feeling of fetal movement. The two have two other children, ages 7 and 11.

After running for two hours Sunday, Staine had a sore back. On Monday, the pain became unbearable. An ambulance rushed her to a Duluth hospital, where she and her husband were stunned by the news: She was about to have a baby girl.

"This is highly unusual," said Dr. Elizabeth Elfstrand, an OB-GYN at Abbott Northwestern Hospital in Minneapolis, adding that in her 20-year career, she's never delivered a baby where the woman didn't know she was pregnant until going into labor.

But while the phenomenon is rare, it does happen occasionally — and usually creates quite a buzz. A short-lived cable reality show called "I Didn't Know I Was Pregnant" followed women who found out they were pregnant when they went into labor.

It's happened to Dr. Diane Horvath-Cosper seven times in her seven years delivering babies at Regions Hospital in St. Paul. Usually the women come into the emergency room for back or abdominal pain, she said — only to find out after an ultrasound that they're in labor.

"I think people look at it and say, 'How can that possibly happen?' I can assure you it does," Horvath-Cosper said.

That's because some women may not experience the common symptoms of nausea or muscle tenderness, and even more rare, they may not experience the baby kicking or, if they do, may dismiss it as muscle spasms. And if a woman wasn't having regular menstrual periods, she might not notice the loss of a period.

"There's a wide range of what's normal in a normal pregnancy," Horvath-Cosper said.

According to the U.S. Centers for Disease Control and Prevention in Atlanta, a failed vasectomy results in fewer than one pregnancy per 100 women in a year.

But the biggest factor for surprising pregnancies like Staine's could be relatively simple: the power of the mind.

"Denial is quite strong," Horvath-Cosper said. "They just convince themselves they're not pregnant. … Usually people make the best of it."

That's what the Staines are doing. They named their baby girl Mira — short for Miracle.

Star Tribune staff writer Kelly Smith contributed to this report.