What are the Chances of a Vasectomy Failing?

Once they are certain that they do not want to have children, many people start looking for effective, long-term birth control solutions. Male sterilization, called vasectomy, is a permanent form of birth control. During a vasectomy, a man's vas deferens is cut, tied, cauterized, or blocked off to prevent sperm from entering his ejaculate during sex. The vasectomy procedure is quick and recovery is also swift, with most men returning to their normal routines within a week. However, vasectomy is not always perfect, and sometimes men are still able to impregnate their partners after the procedure.

Sperm Survival and Leakage

Within about three months after the vasectomy, men must return to their doctors to have their sperm count checked, and to verify that their ejaculate is free of sperm. At this office visit, some men may still have sperm in their ejaculates. Sperm are able to live in the tiny tubes of a man's reproductive system for several months after vasectomy. Doctors recommend that couples avoid unprotected sex during this time, or pregnancy may result.

After the first few months, a primary reason for vasectomy failure is when sperm leak out of the cut end of the vas deferens, create a new channel, and enter a man's reproductive system and ejaculate. This is very rare, but it can lead to pregnancy. A study published in 2004 in the journal "BioMed Central Urology" found that vasectomies using cauterization to close the vas deferens have higher rates of long-term success. Cauterization uses very high temperatures to seal the ends of the vas deferens closed, which may reduce the chances of sperm leaking out of the vas.

Chances of Failure Between Six and 72 Weeks

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Researchers from the CDC questioned over 500 fertile women about their experiences with pregnancy after their partner had a vasectomy 1. The women were contacted periodically over five years. Out of 540 women, six pregnancies occurred within five years, all between six and 72 weeks after vasectomy. Three conceptions took place within 10 weeks of the vasectomy, indicating that sperm were still viable in the man's system. The other three pregnancies occurred at 20, 66, and 72 weeks post-vasectomy. The researchers concluded that the failure rate for every 1,000 vasectomies was 9.4 percent during the first year, and 11.3 percent for years two to five years.

Chances of Failure After 72 Weeks

In the CDC study, no pregnancies were reported between 72 weeks and five years post-vasectomy. The CDC authors acknowledge that vasectomies do fail after five years, although they did not observe it in their study population. Overall, vasectomies are most likely to fail in the first year after the procedure, usually within the first few months.