The problem is that no matter how many people on MN relationship boards talk about 'skin crawling', 'fucking self respect', 'sti checks' and needing to be honest, a whole load of people are unfaithful and, as a species, we will continue to be.
Many, many women have passed other men's children on as their DH's. This is a fact. Recently, in a murder investigation in Bergamo, everyone had to be DNA tested and they found 5 such children in two tiny villages. The suspected murderer is another.
If she still wants to stay with your husband, i'd stay schtum and have a termination. OP, you know it was wrong, but I don't think your DH needs to know, nor the lover. Don't let all your family be punished for your very large mistake. You could get found out, and the consequences would be huge, but if you truly bear the guilt alone, chances are you won't be. I wouldn't underestimate how hard that will be.
I have never slept with anyone other than my husband, since we married, btw.