What to do

When single I know it can be common to have hook ups, however. What do u do in a situation where something doesn’t get reveled til later on or even years later. Like how is the best way to go about it, if u find out that when u were both single that your boyfriend/husband had a kid with someone and had no idea about it til a letter from child support comes in the mail or a letter requesting DNA. However requesting DNA doesn’t make it official. It has to be proven. And what happens if those request letters r ignored?