Antonio Cromartie case raises question: How often do vasectomies fail?

New York Jets cornerback Antonio Cromartie is expecting twins with his wife Terricka despite having undergone a vasectomy.

The couple -- who already have two children together -- told US Weekly that the news came as a shock when Terricka learned she was pregnant in October after a visit to the emergency room for bad cramps.

"I didn't even tell Antonio right away because I didn't think it was possible," she told the magazine. "I was going back and forth in my head how it could even happen." The NFL player also has eight other children from previous relationships.

A vasectomy is a form of birth control for men that is meant to be permanent, and doctors say it is normally very reliable.

"It's an in-office procedure and takes about 10 minutes to do," Dr. Natan Bar-Chama, director of male reproductive medicine and surgery at Mount Sinai Hospital in New York City, told CBS News. "If someone feels that their family is complete, this is certainly a good option."

The procedure involves the closure or blocking of the vas deferens -- the tubes that carry sperm. When the tubes are closed, sperm cannot leave the body and cause pregnancy. In the months after the operation, a patient's semen is tested to make sure it does not contain sperm.

According to the National Institutes of Health, sperm count gradually decreases and it takes about three months for sperm to no longer be present in semen. A second form of birth control is needed to prevent pregnancy until it is confirmed the semen is sperm-free.

It is unclear from reports when Cromartie had the procedure done.

In very rare instances, the vas deferens can grow back together and pregnancy may occur. This happens in about 1 in 1,000 cases, according to Planned Parenthood's website.

Bar-Chama also said that there's a small chance an operation could be performed incorrectly. "Could a doctor have not done the procedure properly and the follow up semen analysis didn't confirm sterility? Sure, it's possible," he said. "But if effectively done properly, it should not fail."

The vast majority of times when pregnancy occurs after a man has undergone a vasectomy, Bar-Chama said, it's because the man has changed his mind and sought medical help to have another child. In this case, the man would have two choices.

"One option is to do reconstructive surgery. That's a microscopic procedure that takes two to four hours and at that point we put together the two ends of the vas deferens and enable the couple to conceive naturally," Bar-Chama said.

The second option, he explained, is to do an in vitro procedure where sperm is extracted from the testicles with a thin needle and injected into a woman's retrieved eggs. When the embryos develop -- usually one or two depending on egg quality and the woman's age -- they are put back into the female with the hope of resulting in a successful pregnancy.

"In each case, we sit down and talk and present both of those scenarios to see which of those best suits that couple," he said.

Despite what they call a surprise, the Cromarties say their pregnancy is a blessing. "It was shocking news for the both of us. It took me a while to process it, but Antonio stood firm and was saying, 'It's God's will.' And he's been excited the whole time," Terricka said. She told Us Weekly she plans to get her tubes tied after the twins are born.

The couple is reportedly working on a book about blended families and lobbying for a reality show.