Twins born to a surrogate mother are deprived of their right to be part of a constitutional family while the law fails to recognise their biological mother, the High Court has heard.

The submission was made by lawyers for a woman who has taken legal action to be registered as the mother of children born to her sister using her embryos.

Gerard Durkan SC said the State's failure to recognise the genetic mother who provided the embryo with her husband was a failure to protect and vindicate their constitutional rights to form a legal family.

He said in opposing their application the State was effectively asking the court to "ignore the biological truth" and "tear up a scientific test" required in legislation to establish parentage.

The children were entitled to the protection and security of a legal family, regardless of how they were conceived and born, Mr Durkan added.

He said the genetic mother in this case had provided the egg and fulfilled the role of mother in every aspect since birth.

For registration purposes parentage for males turned solely on genetic factors, but for females it did not and that created a discrimination.

He added there was no justification for this discrimination.

Mr Durkan said under the Status of Children Act any child may go to court to seek a declaration of parentage and the test for this is a DNA test.

The legal basis on which the Oireachtas had decided parentage to be established was based on inheritable characteristics established by DNA testing.

That was the test, regardless of whether it was a mother or father or the circumstances of conception and birth, he said.

Mr Durkan said the Oireachtas could have chosen a different regime and excluded mothers from a simple DNA test.

It could have said only women who gave birth could be declared a mother but it had chosen not to.

That, he said, was the regime in force in Ireland.

He said were the tests to be applied to the family at the centre of this case the sister would be excluded as the mother, yet she was the legal mother according to the register of births.

Mr Durkan said the maxim applied by the Chief Register of "Mater Certa Semper Est" - motherhood is always certain - was of little use now in the context of scientific advancement and in the context of the regime in place under the Status of Children Act.

He said it was "always a good idea if the law reflected the reality".

The principle of the gestational mother being presumed to be the mother could be treated as a rebuttable presumption as it is for married fathers.

This solution would not "do any violence" to the law as it stands, he said.

Mr Durkan said this solution was merely a refinement of procedures and was not moving away from where the law was.

He said the State was asking the court to "tear up the scientific test" to say even in the absence of inheritable characteristics the gestational mother was to be treated as the mother in law even though there was no legislation to underpin it.

The State would have the "biological truth ignored" even though there was legislation requiring biological tests to establish parentage, he said.

Lawyers for the State will begin making final submissions tomorrow.