When a man gets a vasectomy, you assume he’s no longer able to have kids. But New York Jets cornerback Antonio Cromartie and his wife Terricka discovered that’s just not the case. Terricka gave birth to twins on Sunday, after a pregnancy that came as a complete shock to the couple, given that Antonio had had a vasectomy.

Terricka tells Us Weekly that she discovered she was pregnant when she went to the ER with bad stomach cramps in October. “I really thought I was dreaming, even after, I was just in total disbelief,” she says. “It took me a while to process it.”

It sounds surprising, but experts say it happens more often than you’d think.

Dr. Kevin Campbell, a board-certified urologist at The Urology Group, tells SELF that he always gives patients a heads up that vasectomies aren’t foolproof.

“We counsel people that the failure rate for a vasectomy is one in 2,000,” he said.

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While you’re probably aware that men typically get a vasectomy to avoid having more children, you may not know how it all works. In a vasectomy, a doctor will cut and seal part of the vas deferens, a duct that transports semen from the testicles to the prostate, which eventually lets sperm go out through the penis, Campbell explained. Then, what should happen is that sperm isn’t in a man’s ejaculate.

Dr. Marc Leavey, an internist at Baltimore’s Mercy Medical Center, tells SELF that vasectomies have the greatest risk of failing soon after a man has the procedure done.

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“After a vasectomy, it may take several months, and dozens of ejaculations, to clear all of the sperm from the system,” he said. “Having sex before the sperm count is reliably zero can clearly result in a pregnancy.” 

Campbell says his practice recommends that men don’t ejaculate for a week after surgery and then “strongly recommend” that men wait two months before having unprotected sex.

“When we perform a vasectomy, we take away the ammo, but the gun’s still loaded,” he explained. “You have these storage chambers inside that are already filled with ejaculate and it takes a while for it to get out of the system.”

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Dr. Brian Norouzi, a urologist with California’s St. Joseph Hospital, told SELF that vasectomies can also fail when two sections of the vas deferens grow back together, a process called “recanalization.”

Urologist Dr. David Kaufman, of New York’s Central Park Urology, told SELF that it’s important for men to have a semen analysis about three months after a vasectomy to make sure that he’s not still ejaculating sperm. If he is, he should wait longer to have unprotected sex. And, while rare, a repeat vasectomy may be necessary in some cases, Norouzi said.

Despite the chance that a man could still have a child after a vasectomy, Campbell points out that the procedure is better than any other form of permanent sterility in terms of success rates.

“One in 2,000 is the best odds you can have other than abstinence or a hysterectomy,” he said.

By the way, Terricka said this is definitely her last pregnancy—she plans to have her tubes tied.