Mater semper certa est
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Mater semper certa est ("The mother is always certain") is a Roman-law principle which has the power of praesumptio iuris et de iure, meaning that no counter-evidence can be made against this principle (literally: Presumed there is no counter evidence and by the law). Its meaning is that the mother of the child is always known. 
Since 1978, when the first child was conceived by the technique of in-vitro fertilization, the principle of "Mater semper certa est" no longer applies, since a child may have both a genetic and a natural ("birth") mother. Since then some countries have converted the old natural law to an equivalent codified law - in 1997 Germany has introduced paragraph 1571 "Mutterschaft" (motherhood) of the BGB (civil code) reading "Mutter eines Kindes ist die Frau, die es geboren hat." (mother of a child is the woman who has given birth to it).
The Roman law principle however does not stop at the mother, in fact it continues with "pater semper incertus est" ("The father is always uncertain"). This was regulated by the law of "pater est, quem nuptiae demonstrant" ("father is to whom marriage points"). Essentially paternity fraud had originally been a marriage fraud in the civil code (in Germany the historic "Ehelichkeitsanfechtungsklage" was simply renamed as "Vaterschaftsanfechtungsklage" when legal paternity was redefined) due to this principle. Today even married fathers will often use the modern tools of DNA testing to ensure a certainty on their fatherhood.
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