A baby could become the first person born in England or Wales who will not legally have a mother if a transgender man wins a historic human rights battle.
The youngster is the child of a single parent who was born a woman but now lives as a man after undergoing surgery.
Lawyers have told a judge that he had been biologically able to get pregnant and give birth but had legally become a man when the child was born.
They say the man wants to be identified as the child's "father" or "parent" on a birth certificate.
But they said a birth registrar has told the man the law requires people who give birth to children to be registered as mothers.
The man has taken legal action against the body set up to administer statutory provisions relating to the registration of births and deaths after complaining of discrimination.
He says forcing him to register as the child's "mother" breaches his human right to respect for private and family life.
The man says such "interference" is not proportionate or necessary in the light of changes which have "evolved in society".
A judge on Thursday heard preliminary argument from lawyers representing the man and lawyers representing The Registrar General for England and Wales at a High Court hearing in London.
Mr Justice Francis said the issue had never been raised in a court in England and Wales before.
He said if the man won his fight ministers might have to consider changing the law.
A judge is due to make a decision after a trial in September.
Mr Justice Francis heard preliminary discussion at a public hearing but he said the baby could not be identified in media reports.
The judge said reports should not reveal the baby's gender or age, or the man's age, and should not give any clue where the man and the child lived.
Lawyers say other transgender men have given birth but have been registered on birth certificates as mothers.
They say if the man wins his fight the child will be the first person born in England or Wales not to legally have a mother.
The man held the baby as he sat next to a lawyer at Thursday's hearing.
At one point he stood to rock the baby, who sucked on a dummy, to sleep as lawyers discussed issues with the judge.