What are the chances of pregnancy after a successful vasectomy?

Patient: What are the chances that a baby is conceived after a successful vasectomy? The man got his sperm tested after the positive pregnancy test and said there were no sperm seen. Is it possible that one could have slipped through a month prior?

Doctor: A vasectomy is a very effective method of birth control with a documented failure rate of 1 pregnancy per every 2000 vas ectomies. However, pregnancy is still a possibility if there is sexual intercourse shortly after the procedure i.e within approximately two months. Most surgeons suggest checking the sperm count twice after the procedure to ensure that it is zero before engaging in sexual intercourse without some other method of contraception.