What is the probability of a man who had a vasectomy fathering a child?

Answer

A vasectomy is simply the cutting and tying back of the vasa deferentia. The procedure has a failure rate of around 0.1% to 0.2% for the first year. Assuming he and the partner are otherwise normal and do not use any contraception, there is an 85% probability of pregnancy during the same time. So there is roughly a 0.15% chance of pregnancy occurring (in the first year after vasectomy).