Updated November 2018
After a vasectomy, it can take many months before your semen is free of sperm. During this time, you will need to continue using another form of birth control while having sex.
Anatomy of a Vasectomy
Sperm travels from the testicles through several tubes before they are mixed with the semen. One of these tubes is the vas deferens, which runs from a structure called the epididymis on each testicle. During a vasectomy, both vas deferens are cut. This prevents new sperm made in the testicles from being ejaculated along with the semen.
However, after a vasectomy, some sperm still remain in the part of the vas deferens closer to the penis—the upper part. It can take 20 to 30 ejaculations before all the sperm are cleared from these tubes. In most men, it may be three or more months before they have no sperm—or very few non-motile (non-moving) sperm—in their semen.
It only takes one sperm to fertilize a woman’s egg. So until the vas deferens is free of motile sperm, there is still a chance of pregnancy. During this time you will need to use another type of contraception every time you have sex.
Your doctor will retest your semen about eight weeks after your vasectomy to see if there are still sperm present. This shows whether the vasectomy was successful, meaning you are infertile. If the tests show that your semen is free of sperm or only a few dead ones remain, then you no longer have to use additional birth control.
Some men, though, will continue to have a small number of non-motile sperm in their semen. Because the sperm is non-motile, the risk of pregnancy may be low but may require follow-up testing.
One study found that around 10 percent of men undergoing a vasectomy reversal still had non-motile sperm in their semen. However, no pregnancies occurred in the partners of these men since their vasectomy.
If sperm is still present in your semen during the follow-up test, your doctor will talk to you about your options. This may include further testing. Your doctor may also decide that the chance of pregnancy is low enough that you don’t need to worry.
A vasectomy failure means that sperm continue to be present in the semen after a certain amount of time has passed since the vasectomy. This can happen during the period before the vas deferens tubes were cut. In rare cases, men may have two vas deferens on one side, and one of these is missed during the surgery.
In other cases, the two cut ends of the vas deferens can rejoin to form a complete tube. Your doctor may suspect this if you continue to have motile sperm after a vasectomy, or your sperm count continues to increase.
When the vasectomy is a failure, men will either have to continue using another form of contraception or undergo another vasectomy.