Vasectomy is one of the most effective forms of birth control. In the first year after a man has a vasectomy, a few couples will still get pregnant. But the number is far lower than the rates of pregnancy among couples using condoms or oral contraceptive pills.1
However, a vasectomy is not effective right away. Men still need to use other birth control until the remaining sperm are cleared out of the semen. This takes 15 to 20 ejaculations, or about 3 months. Even then, 1 of every 5 men will still have sperm in his semen and will need to wait longer for the sperm to clear.2
A health care provider will check a man's semen for sperm at least once after the surgery. Once the sperm count has dropped to zero, it is safe to assume that the vasectomy is now an effective form of birth control.2,3 Until that time, men need to use another form of birth control to make sure their partner does not become pregnant.