Mater semper certa est

 ("The mother is always certain") is a Roman-law principle which has the power of , meaning that no counter-evidence can be made against this principle (literally: presumption of law and by law). It provides that the mother of the child is conclusively established, from the moment of birth, by the mother's role in the birth. 
Since egg donation, or embryo donation with surrogacy, started using the technique of "in-vitro" fertilization, the principle of has been shaken, since a child may have a "genetic" and a "gestational" ("birth"), let along a "social", mother who are different individuals. Since then some countries have converted the old natural law to an equivalent codified law - in 1997 Germany introduced paragraph 1571 ("motherhood") of the BGB (civil code) reading ("the mother of a child is the woman who gave birth to it"). This has also been tested in the British case of Freddie McDonnell 
The Roman law principle however does not stop at the mother, in fact it continues with ("The father is always uncertain"). This was regulated by the law of ("the father is he to whom marriage points"). Essentially paternity fraud had originally been a marriage fraud in the civil code due to this principle. Today some married fathers use the modern tools of DNA testing to ensure a certainty on their fatherhood.
References.
https://www.pinknews.co.uk/2019/09/25/freddy-mcconnell-trans-man-gave-birth-father-high-court-legal-battle/