Does the other man know you are pg? If you can tell him before DH then do so, and suggest that he do a DNA test. I do think that the 2 of you need to work out the best way to tell DH and his DF, because it will only hurt you more to keep a secret like this from the man you love. Yes it will cause a lot of pain in the short term, but long term it could be much worse. Honestly if my DH had slept with someone else I would rather know and then get on with learning to rebuild trust than find out down the track. The sooner you tell him the easier it will be for him to rebuild trust if it is able to be rebuilt. If the trust can't be rebuilt then it is not any better to keep him in the dark about it because it will knaw away at you and may make it hard for you to trust him, even if he has nothing to hide.