A decision by the High Court declaring the genetic mother of twins born to a surrogate as the legal mother was "radically wrong" and attempted to reverse the meaning of motherhood, the Supreme Court has been told.

Seven judges of the Supreme Court have begun hearing an appeal by the State against the landmark judgment handed down last year.

Lawyers for the State told the court the transfer of parentage could not be done "by private contract" but must be done by a process of public law such as adoption.

They said the birth mother was the mother as a matter of public law and no amount of arrangement between a genetic and birth mother could change that.

They also say the rights of a surrogate mother would be "switched off" at the moment of birth by the decision of the High Court.

Senior counsel Michael McDowell said it was a matter for the Oireachtas to change the law.

He said the definition of motherhood under public law was, and is, that the woman who carries the child to birth is the mother.

If someone else is to become the mother it cannot be by a private contract, he said, adding: "You can't have two mothers at the same time."

The case came before the High Court last year when the mother of twins born to a surrogate sought a declaration from the High Court that she was the legal mother of the children and could be named on their birth certificates.

The woman was born without a womb but her sister agreed to carry embryos made by the woman and her husband at a fertility clinic.

Mr Justice Abbott ruled that maternity could be established on the basis genetics and DNA, as it is for paternity, and said the woman had the right to be recorded as the legal mother on the birth certificate.

On the opening day of the State's appeal to the Supreme Court, Mr McDowell said the decision was radically wrong and gives rise to very serious difficulties for the interpretation of the Constitution and seriously compromised the capacity of the Oireachtas to deal with the issue of surrogacy and ascribing parentage.

He said if the High Court decision was allowed to stand, motherhood would be ascribed to the donor of an egg.

The consequences of preferring genetic motherhood to birth motherhood regarding the public law of the country and the rights of the individual involved were "massive", he added.

He said this was a classic case of hard cases making bad law.

The High Court decision could not be devoid of consequences for many other women who had used donated ova to have children.

This decision would open up an appalling situation where they would not be the mothers or guardians of their own children.

The rights of surrogate mothers would be "switched off at birth" under this judgment, he added.

He said the question had been asked if the Constitution contained a definition of motherhood but that may be an unanswerable question.

He said on any standard maternity always involved pregnancy at the very least.

Parenthood could be transferred from one person to another such as in adoption but this required a public law process.

The High Court had found adoption was an option for the family at the centre of this case, he added.

He said the State was seeking to have the High Court judgment overturned because the law had not changed.

The genetic mother had no more rights than "a sperm donor from California" who comes looking for their offspring and asserting constitutional rights over children born using their sperm.

Science may have moved on but the law had not changed, he said.

He said there was nothing irrational or unfair to distinguish between maternity and paternity and saying one is very, very different from the other and has very different legal consequences.

Ireland was by no means an outlier when compared to other European countries by holding the birth mother as the legal mother as a matter of public law, he said.

Mr McDowell said draft legislation published last week was attempting to deal with the issues surrounding surrogacy.

Under the proposed new law, the commissioning parents would be deemed to be the parents, he said.

The hearing is expected to last three days.