Sometimes the paternity of a child becomes a factor in child custody cases when the mother goes to court for child support of a child born out of wedlock.

Unlike maternity, which is never uncertain from a legal standpoint, paternity can be in doubt and give rise to a paternity suit, which is a legal action to determine whether a man is the father of a child. The purpose of this action is to enforce support obligations.

Paternity estoppel comes into play when 1) one party misrepresents paternity and 2) makes that misrepresentation with actual and constructive knowledge of the truth concerning paternity; 3) when the misrepresentation is made in a way that is "both natural and probable that it will be relied upon by the other party or the child"; 4) when either the presumed father or child relies upon it in a way that 5) "was reasonable under the facts"; and 6) that reliance resulted "in injury."

Courts hold men to a strict liability for a child that results from their sperm. A man cannot enter into consent agreement with a woman to impregnate her, and then contend that he is but a sperm donor without a responsibility to support the child. Nor can the woman who becomes pregnant as a result of such an agreement waive child support. Courts have ruled that, outside of the jurisdiction's statute on artificial insemination, a man cannot waive his parental rights (or the responsibility of child support), nor can a mother. To do so goes against public policy.

Technological advances such as DNA testing have made paternity a question of science, not law. These advances have undermined Lord Mansfield's Rule, which render inadmissible the testimony of either spouse as to whether a husband had access to his wife at the time of conception. In some jurisdictions, Lord Mansfield's rule has been abandoned.