What is the Sharia ruling on establishing the lineage of a fetus that may have resulted from an illicit relationship involving my wife, and can medical analyses be relied upon to establish its lineage?

The default rule is that what a wife delivers belongs to her husband, and paternity is only disproven through li'an (oath of condemnation). Medical analyses do not take the place of li'an. The Council of the Islamic Fiqh Academy has issued a resolution confirming the impermissibility of relying on DNA fingerprinting to negate paternity, and that it does not substitute for li'an. It is permissible to rely on it in criminal investigations for some crimes, and in establishing paternity in cases of unknown parentage, suspected mixed-up newborns, and lost children.