Medical Mystery Behind Mom's Surprise Birth

April 9, 2010— -- Six years ago, Ryan Emmons of Pierson, Mich., had a vasectomy. With three kids, the 32-year-old welder at an automotive parts company decided with wife Carri that their family was complete.

But last April the unimaginable happened when Carri, 27, woke up in the middle of the night in pain and quickly went into labor in the family bathtub. "Oh, my God, I'm looking at a baby! ...I didn't know my wife was pregnant," a frantic Emmons told the 911 dispatcher.

Moments later, Carri gave birth to a baby boy, William Gerald Emmons.

Both Emmons and Carri were shocked. "The possibility of her even being pregnant is not even an option because I was supposed to be done," he said.

Vasectomies are one of the most commonly performed procedures done in the U.S. With a success rate of over 99 percent, doctors say they're the most reliable form of birth control that exists.

With that security, Carri -- already a mother-of-three -- thought it was impossible for her husband to impregnate her again. She told "20/20" she was worried Ryan would think she had cheated.

"I think that's kind of a normal reaction, you know, for him to think that as a spouse," she said. "If I were him, I would question."

Click here for more on Carri's surprise pregnancy

Carri's family soon learned of the surprise pregnancy. "My first reaction... And then it sank in that, 'Wait a minute, Ryan has a vasectomy. How could this happen?'" Carri's sister Kelly Sabin recalled.

The answer: a failed vasectomy.

"Even though vasectomy is now the most effective method of male contraception, it's not 100 percent effective," said Daniel Williams, assistant professor of urology and director of male reproductive medicine and microsurgery at University of Wisconsin-Madison School of Medicine and Public Health.

According to Williams, there are two main categories of vasectomies that don't work.

"One would be a vasectomy that after the procedure, there's still sperm present in the semen... It's usually due to an error of surgical technique," Williams explained.

The second occurs through a process known as re-cannalization, where the small tubes that are sealed off during a vasectomy spontaneously reconnect. It happens in less than one percent of all procedures and the cause is unknown.

"One potential cause of a late or early re-cannalization is something called a sperm granuloma, which occurs when a sperm leaks out of the vasectomy site. There can be a risk of sperm falling out and the sperm reconnecting," Williams said. "Fortunately, it's extremely rare."