A vasectomy is a surgery that prevents pregnancy by blocking sperm from entering semen. It’s a permanent form of birth control. It’s a pretty common procedure, with doctors performing more than 500,000 vasectomies per year in the United States.

The procedure involves cutting and sealing off the vas deferens. These are two tubes that carry sperm from the testicles to the urethra. When these tubes are closed off, sperm can’t reach the semen.

The body continues to produce sperm, but it’s reabsorbed by the body. When someone with a vasectomy ejaculates, the fluid contains semen, but no sperm.

Vasectomy is one of the most effective birth control methods available. But there’s still a very small chance that the procedure won’t work, which could result in pregnancy. Even if a vasectomy is completely effective, it can take some time for this method to start protecting against pregnancy. There may still be sperm in your semen for a few weeks afterward.

Read on to learn more about pregnancy after vasectomy, including rates and reversal options.

There aren’t any standard odds of getting pregnancy after vasectomy. A 2004 survey suggests that there’s about 1 pregnancy per every 1,000 vasectomies. That makes vasectomies about 99.9 percent effective for preventing pregnancy.

Keep in mind that vasectomies don’t offer immediate protection against pregnancy. Sperm are stored in the vas deferens and will remain there for a few weeks or months after the procedure. This is why doctors recommend that people use an alternative method of contraception for at least three months after the procedure. It’s estimated that about 20 ejaculations are required to clear out all the sperm. Learn more about having sex after vasectomy.

Doctors also usually have people who’ve had a vasectomy come in for semen analysis three months after the procedure. They’ll take a sample and analyze it for any live sperm. Until this appointment, it’s best to use a backup birth control method, such as condoms or the pill, to prevent pregnancy.

In a small percentage of cases, pregnancy can occur even after having the procedure. This is usually due to not waiting long enough before having unprotected sex. Not following up with a sperm analysis appointment is another common cause.

A vasectomy can also fail a few months to years later, even after you’ve already had one or two clear semen samples. This can happen because:

  • the doctor cuts the wrong structure
  • the doctor cuts the same vas deferens twice and leaves the other one intact
  • someone has an extra vas deferens and the doctor didn’t see it, though this is rare

Most often, the surgery fails because the vas deferens grows back afterward. This is called recanalization. Tubelike cells begin to grow from the cut ends of the vas deferens, until they create a new connection.

A 2018 study found that just over 7 percent of people who’ve had a vasectomy end up changing their mind. Luckily, vasectomies are usually reversible.

A vasectomy reversal procedure involves reconnecting the vas deferens, which allows sperm to enter the semen. But this procedure is more complicated and difficult than a vasectomy, so it’s important to find a skilled surgeon.

There are procedures that can reverse a vasectomy:

  • Vasovasostomy. A surgeon reattaches the two ends of the vas deferens using a high-powered microscope to view the tiny tubes.
  • Vasoepididymostomy. A surgeon attaches the top end of the vas deferens directly to the epididymis, which is a tube in the back of the testicle.

Surgeons usually decide which approach will work best when they’re starting the procedure, and they may opt for a combination of the two.

The Mayo Clinic estimates that the success rate of vasectomy reversals is between 40 and 90 percent, depending on a range of factors, such as:

  • how much time has passed since the vasectomy
  • age
  • partner’s age
  • surgeon experience

Vasectomy is very effective at preventing pregnancy, but it’s also permanent. While pregnancy after vasectomy is possible, it’s pretty rare. When it does happen, it’s usually the result of not following postsurgery guidelines or a surgical mistake.

Vasectomies can also be reversed but it can be a more complex procedure. Talk with your doctor if it’s something you’re looking to consider.