What are the chances that a vasectomy could “reverse itself” and allow a pregnancy to occur? This is one of the top questions men (and women) have about the vasectomy procedure.

The short answer is that even though it’s technically possible for a vasectomy to reverse spontaneously, it is extremely rare.

How can a vasectomy reverse itself?

An unintended post-vasectomy reversal is known as recanalization. Recanalization is a process in which sperm regains the ability to enter a man’s semen after a vasectomy, resulting in the remote possibility of pregnancy.

Harvard Medical School reports that recanalization occurs in approximately 1 in 4000 vasectomies. This figure will vary from source to source, but the bottom line is that recanalization is a very uncommon event.

Even when recanalization does occur, the resulting sperm counts are typically much lower than before and are less likely to result in a pregnancy. Just because a man has had a recanalization doesn’t mean he will have pre-vasectomy levels of fertility.

Another consideration is that most studies on recanalization and vasectomy failure were performed before the advent of new surgical techniques such as thermal cautery and fascial interposition. These new methods have further increased the effectiveness of vasectomy and have brought the recanalization rate even closer to 0%.

How does recanalization happen?

When a vasectomy has “reversed itself” or recanalized, it doesn’t mean that open ends of the vas deferens tubes have simply reconnected. The surgical techniques involved with modern vasectomy procedures–along with the body’s natural healing and scarring processes–make this more or less impossible.

A recanalization occurs when sperm are able to escape from the cut end of the vas deferens due to microscopic channels that have formed in the patient’s scar tissue. These tiny channels may be the result of a sperm granuloma, improper surgical technique, or other complications. If these microchannels develop and the two severed ends of the vas deferens are very close or touching, sperm may be able to travel from one section of the tube to the other.

For more information on this topic, see our article on post-vasectomy recanalization.

Risk of pregnancy after recanalization

The newly formed microchannels resulting from recanalization are much smaller than a man’s previously intact vas deferens, so the chances of pregnancy will probably be greatly reduced compared to his pre-vasectomy fertility levels.

Nonetheless, recanalization is a form of vasectomy failure, which means the patient is not considered sterile. In situations where recanalization has occurred, an additional vasectomy will be necessary to ensure 100% sterility.

How is recanalization detected?

Recanalization can be detected during the post-vasectomy semen analysis, which usually takes place three months after the procedure. Unfortunately, many men do not return for a sperm count, and in these cases, a recanalization is not discovered until a man’s partner has an unexpected pregnancy.

This is one reason why it is so vital for men to follow their doctor’s follow up instructions and have a proper sperm count after their operation.

Other types of vasectomy failure

Not all vasectomy failures are a result of recanalization. If a man has unprotected sex before semen analysis confirms his sperm count is zero, he can still impregnate a woman. For more on this, see our article on pregnancy after vasectomy.