Reciprocal in vitro fertilisation (RIVF) allows two women to share in the process of childbearing -- one woman's egg, fertilised externally with the aid of a sperm donor, is transferred to the other who carries the pregnancy to term.

The couple, who were granted anonymity by the court, had already won a 2023 legal bid so that the woman initially denied legal status was recognised as a "parent at common law".

The judge ruled at the time that the city's family laws were a form of discrimination.

However, the Department of Justice refused to re-register the birth, with officials saying there was "no legal basis whatsoever" for the change -- prompting the couple to sue the government again.

On Tuesday High Court judge Russell Coleman ruled not having both women listed on the birth certificate would "seriously interfere" with the child's rights.

He cited the example of administrators seeking a parent's decision in urgent medical treatment.

The absence of official registration for one of the women "will at least likely cause real doubt as to whether she is a parent within the family unit", he said.

The child would experience "some inconvenience, embarrassment, and potential harm to his dignity" as a result, he added.

The judge asked both sides to make further arguments as to the remedy and to come up with a timetable.

The court had recognised that the child's rights to equal protection were infringed and such a declaration was "helpful", the couple told AFP in a statement.

RIVF was introduced in the late 2000s and can now be performed without restriction in more than a dozen European countries, according to an academic survey.

As Hong Kong does not recognise same-sex marriages, the two women in the case were married and underwent RIVF in South Africa.