The 27-year-old, who cannot be named for legal reasons, was arrested at Heathrow Airport under terrorism laws when she landed from Addis Ababa, Ethiopia, earlier this month, 'The Sunday Times' reported.
Her two-year-old son, whose nationality remains unclear, has been taken into the care of the state.
Scotland Yard has released the woman on bail while it continues its investigation.
The UK authorities are working on a series of measures to tackle cases of such ISIS brides and children returning to the UK following the terrorist group's defeats in its Syrian and Iraqi strongholds last year, the report said.
Aqsa Mahmood, a 22-year-old Pakistani-origin suspected ISIS recruiter from Glasgow, has been stripped of her British citizenship to prevent her returning to Britain.
The UK Home Office argues that Mahmood has not been made "stateless" because she is eligible to apply for citizenship in the country where her parents were born, which is Pakistan.
A similar order has been made against another British woman who fled Syria at the end of 2016 after her husband, a prominent British figure in ISIS, died in a battle.
The woman, who cannot be named for legal reasons, gave birth to two children, now aged one and three, in the war zone, which means they have no nationality, the report said.
All three remain stranded in Turkey.
The UK's social services departments have been drawing up plans to take into care British children known to have travelled to Syria and Iraq with their families, should their parents bring them back home.
More than 100 British women are thought to have travelled to Syria and Iraq to join the terror groups.