Wis. Stat. § 891.41: Presumption of paternity based on marriage of the parties.

(1) A man is presumed to be the natural father of a child if any of the following applies:

(a) He and the child’s natural mother are or have been married to each other and the child is conceived or born after marriage and before the granting of a decree of legal separation, annulment or divorce between the parties.

(b) He and the child’s natural mother were married to each other after the child was born but he and the child’s natural mother had a relationship with one another during the period of time within which the child was conceived and no other man has been adjudicated to be the father or presumed to be the father of the child under par. (a).

(2) In a legal action or proceeding, a presumption under sub. (1) is rebutted by results of a genetic test, as defined in s. 767.001 (1m), that show that a man other than the man presumed to be the father under sub. (1) is not excluded as the father of the child and that the statistical probability of the man’s parentage is 99.0 percent or higher, even if the man presumed to be the father under sub. (1) is unavailable to submit to genetic tests, as defined in s. 767.001 (1m).