May 5, 2004 -- As many as one in 100 vasectomies may fail to prevent pregnancy within five years of the procedure, a new study shows.

Researchers say that the study shows that like other birth control methods, vasectomies are not always 100% effective.

The study showed that six pregnancies were reported among 540 women within six weeks to 1.5 years after their husbands had a vasectomy.

"Couples considering vasectomy should be counseled about the small, but real risk of pregnancy following the procedure and that men are not sterile immediately after vasectomy," write researcher Denise J. Jamieson, MD, MPH, of the CDC, and colleagues.

A vasectomy is considered a permanent form of birth control to make a man unable to father a child. It is a surgical procedure in which the tube that carries the sperm from the testes to the urethra is cut or sealed off to prevent sperm from being released during ejaculation.