In Illinois, paternity is presumed if a couple is married and they have a child during the marriage. Furthermore, if the mother was married or in a civil union for up to 300 days before the birth of a child, paternity is also presumed. If a couple who is not married has a child, specific steps can be taken to prove paternity for legal reasons. | O’Flaherty Law – Learn About Law Legal Articles, Videos & Podcasts with O’Flaherty Law