Legally, a child’s father is the mother’s husband or a man who has been assigned paternity in a court proceeding. Biologic paternity is virtually irrelevant without legal recognition. Courts may choose to transfer legal paternity from one man to another, but they endeavor not to disturb paternity when it will render an otherwise legitimate child a bastard. Physicians should also avoid casting doubts on paternity when a family is already in crisis. Although physicians should not lie to patients, there is enough uncertainty about paternity determinations to allow room for differing interpretations. Even tissue typing can be confused by mutation, the presence of related males among the candidates, or laboratory error.

It is best to avoid questions of paternity unless they are directly relevant to the patient’s medical condition or care. If the father is concerned about possible genetic disease in his child, the information gathering should be directed to the disease, not the paternity. Physicians should decline to test a child to determine paternity unless it is a medical necessity. (This is usually an issue only for certain rare genetic diseases and as a side issue in tissue typing for organ transplantation.) If a court orders paternity testing, the physician must honor the order. But the physician acting as an expert witness in such a case should inform the court of the limitations of the tests and avoid making any unequivocal pronouncements of paternity.

While modern DNA testing techniques make paternity testing much more certain, they do little for the social consequences flowing from such tests. DNA testing is now available and marketed directly to the public. If the reasons for testing are social, not medical, physicians should advise patients to seek this information outside the medical setting. If the issue is legal paternity or child support, the state child welfare agency should be involved.