607f2d02b05d096048485597_man worrying about vasectomy failure

Can A Vasectomy Fail?

Dr. Joe Pazona

I have to admit I’m amused when I hear a guy say, “my wife is pregnant . . . it was an accident.” Really? An accident?

Unless we have to review the birds and the bees lecture from 5th grade, then I’m pretty sure we know how this occurred. Now I’ll give it to you that condoms can break and doses of birth control pills can be missed. I’m less understanding if your “pull-out game” is not strong.

But what if you had a vasectomy, the most reliable, permanent sterilization solution for couples, and still your wife is pregnant? What happened?

Chill-out bro, I know where your head is going. This post is PG-13 so I’m not insinuating this poor guy’s wife had an affair or sultry one-night-stand.

I’m asking, “Can a vasectomy fail?” Unfortunately the answer is yes. Nothing in medicine is 100%.

Let’s explore the different reasons how a vasectomy can fail. This way you can be better prepared if your partner has an unexpected bun in the oven. We wouldn’t want you to accuse your spouse of infidelity or start yelling at your urologist for “cutting the wrong tube” if there’s another plausible explanation.

Here are the top reasons why a vasectomy would “fail” leading to an intended pregnancy:

1. Unprotected intercourse before a sperm check

A good friend of mine several years ago went in for his vasectomy appointment because he and “The Mrs.” had decided on “no more kids!” Before leaving for the appointment, he snuck in one more round of gettin’ busy with his wife. 9 months later, child #3 arrived.

Obviously a vasectomy can’t prevent any bullets that have already left the chamber. What is not obvious to many couples, is that the man is NOT immediately sterile after a vasectomy. It takes time to clear the pipes before your man is shooting blanks.


The American Urological Association (AUA) recommends the post-vasectomy semen analysis 8-16 weeks after the vasectomy. Personally I tell my patients they should perform the sperm check after a minimum of 20 ejaculations AND a minimum waiting period of 2 months.

You may be asking yourself, “why does it take so long for the sperm to clear?” Here’s my best analogy:

  • Think of sperm as the swimmers. The swimmers are driving to the swimming pool (seminal vesicles) where the semen (fluid) is made.
  • They have a 200 mile trip (the vas deferens is actually about 1 foot long).
  • A vasectomy involves putting up a permanent road block at mile marker #10. 
  • It takes about 20 ejaculations, and two months, for all the sperm beyond the roadblock to reach the swimming pool.

Occasionally I’ll have the overachiever who thinks he’s ready for a sperm check because he banged out (excuse the inappropriate pun but this is a urology discussion) 20 ejaculations in 20 days. Too soon. Some swimmers haven’t yet reached the pool.

On the flip side, if work has been busy, and therefore you have not “been busy”, then doing a sperm check after 2 months and 10 ejaculations may not be adequate. Get going on your homework.

All of this is to say the following: the most common reason a vasectomy fails is the man never completed the post-vasectomy semen analysis.

Since nothing changes in the performance department with a vasectomy (e.g. erections, fluid volume, orgasm), you have no way of knowing if the vasectomy “worked.”

So before you and your special someone go au naturale without contraception, perform the semen analysis so you can be certain you are shooting blanks after the vasectomy.

2. Technical vasectomy failure

Despite our best intentions as surgeons, mistakes are made with procedures. Unfortunately in this case a vasectomy mistake can have big consequences for you. Remember a sperm check will prevent a vasectomy failure from becoming a bundle of joy in 9 months.

Regardless, it’s important to understand that the doctor performing the vasectomy, may not have successfully occluded or disconnected BOTH vas deferens. Yes you have two tubes. One from each testicle.

So what was “cut” if the doctor didn’t get the right tube? The vas deferens is one of many tubes running in a cord to and from the testicle. The spermatic cord also contains muscles, nerves, and blood vessels. Don’t worry, your ball won’t fall off if the wrong tube was cut. The good Lord gave you back-up blood vessels in case of an emergency.

Now there are certainly anatomic variations. Some men have only one testicle or one vas deferens. Some men have two vas deferens coming from one testicle. If there’s an extra tube then it may have been missed.

Those rare situations aside, how do we know if the doctor swung and missed on snipping one of your tubes? By the sperm check of course. The AUA guidelines consider a vasectomy a failure if there are >100,000 motile sperm at least 8-12 weeks after a vasectomy.

It’s important to understand that if one of the tubes is still connected, you’ll typically see millions of sperm on the sperm check. Although 100,000 swimmers sounds like a lot, the likelihood of a pregnancy with this many swimmers is really low.

How often can a vasectomy fail due to a technical error? It’s unclear but my best guess is 1-2% of vasectomies will have a technical failure. It happens but not often. 

Just make certain you get the sperm check to make sure the doctor’s “oops” doesn’t lead to an “oops” in the bedroom (are you catching the importance of a sperm check after a vasectomy yet?).

3. Recanalization

This is the fancy doctor term for when your tubes were disconnected, but now they found a way to reconnect naturally. The vasectomy worked initially, but now it failed.

We would define recanalization in the following scenario:

  • Sperm check after vasectomy shows no sperm (azoospermia).
  • Man suspected of impregnating a woman (hopefully it’s his partner and not someone else).
  • Man has repeated the sperm check and there are now millions of sperm.

Lucky for couples, the likelihood of recanalization is 1/2000 vasectomies. I tell my patients that the good Lord intended for this child to be on this earth for some reason or another. Hopefully it’s for a good reason because you now need some more diapers.

Remember that I said nothing in medicine is 100%? Well a vasectomy is 99.95% effective once a man is documented sterile by a sperm check. However, a “miracle baby” is still possible.

4. Inaccurate sperm check

There are other steps in the process that can fail leading to pregnancy. What if you did perform the sperm check, were told you were sterile, and then 6 weeks later your wife was pregnant?

Sperm checks or post vasectomy semen analyses are subject to human error as well. Many urologists check the sperm sample themselves in the office. Personally this brings up too many variables for an inaccurate sperm check.

Urologists are typically seeing 4-6 patients every hour. There are multiple urine and semen samples on the counter, phone calls, and other distractions. The possibilities of mislabeling a specimen or not thoroughly looking at a sample are too high for my liking.

Plus, the last time a urologist was consistently using a microscope was in 1st year of medical school during our histology rotation. I’d rather have someone who’s only job is using microscopes tell me if I’m shooting blanks. Therefore I prefer that patients send their samples to independent labs.

To play devil’s advocate, it would be impossible to definitively prove that the reason for the vasectomy failure wasn’t an early recanalization. That being said, since a vasectomy is 99.95% effective in preventing a pregnancy, human error with the sperm check is the more likely cause if you went from sterile to fertile in the first 6 months after a vasectomy.

Closing thoughts

The decision to have no more, or any children is a personal one. Having a vasectomy is a commitment that most men and their partners don’t take lightly. Once you make that commitment, you want to minimize surprises.

Sometimes surprises occur. I cared for a man who had a vasectomy with another doctor. My patient was told that he was sterile on the sperm check. His wife then got pregnant. Uh oh.

I examined him, performed another sperm check to make certain that the first one was accurate, and . . . no sperm again. Double uh oh. That was a difficult conversation to have.

As any man would, he questioned his wife’s fidelity and had an even more difficult conversation with her. She denied any breach of trust and suggested a paternity test to prove her faithfulness. 

Guess what? The child was his! Somehow, despite TWO NEGATIVE SPERM CHECKS, my patient impregnated his wife again. 

My explanation? I have none. Miracles happen in this world. And sometimes these miracles lead to sleepless nights, minivans (oh heck no) and prom dresses 17 years later. 

But as most parents would tell you, myself included, it’s all worth it in the end.

Life can throw you curveball (vasectomy failure) and sometimes that curveball leads to a grand slam.

But for Pete’s sake, go get that post-vasectomy sperm check already.


Dr. Joe Pazona

Dr. Joseph Pazona is the founder and President of VirtuCare, a telemedicine solution for connecting patients with physician experts.

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