Illinois Statutes Chapter 750. Families § 46/204. Presumption of parentage

(1) the person and the mother of the child have entered into a marriage, civil union, or substantially similar legal relationship, and the child is born to the mother during the marriage, civil union, or substantially similar legal relationship, except as provided in the Gestational Surrogacy Act or other law;

(2) the person and the mother of the child were in a marriage, civil union, or substantially similar legal relationship and the child is born to the mother within 300 days after the marriage, civil union, or substantially similar legal relationship is terminated by death, declaration of invalidity of marriage, judgment for dissolution of marriage, civil union, or substantially similar legal relationship, or after a judgment for legal separation, except as provided in the Gestational Surrogacy Act or other law;

(3) before the birth of the child, the person and the mother of the child entered into a marriage, civil union, or substantially similar legal relationship in apparent compliance with law, even if the attempted marriage, civil union, or substantially similar legal relationship is or could be declared invalid, and the child is born during the invalid marriage, civil union, or substantially similar legal relationship or within 300 days after its termination by death, declaration of invalidity of marriage, judgment for dissolution of marriage, civil union, or substantially similar legal relationship, or after a judgment for legal separation, except as provided in the Gestational Surrogacy Act or other law; or

(4) after the child's birth, the person and the child's mother have entered into a marriage, civil union, or substantially similar legal relationship, even if the marriage, civil union, or substantially similar legal relationship is or could be declared invalid, and the person is named, with the person's written consent, as the child's parent on the child's birth certificate.