[1] Since egg donation, or embryo donation with surrogacy, started using the technique of in-vitro fertilization, the principle of mater semper certa est has been shaken, since a child may have a genetic and a gestational ("birth"), let alone 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 1591 Mutterschaft ("motherhood") of the BGB (civil code) reading Mutter eines Kindes ist die Frau, die es geboren hat ("the mother of a child is the woman who gave birth to it").
[2] 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 ("the father is he to whom marriage points"; see presumption of legitimacy).
Today some married fathers use the modern tools of DNA testing to ensure a certainty on their fatherhood.