Polly McCourt, 39, who was born in Oxfordshire, returned to her apartment on Manhattan's Upper East Side after feeling unwell at a nearby school fundraiser. She phoned her doctor, who advised heavily pregnant Polly to go to hospital.
She phoned her husband Cian, who was stuck in traffic, and then headed downstairs to get a taxi. But her waters broke before she made it out of the building and by the time she made it onto the street she was already in labour.
Even more shocking, the taxi Polly flagged down was pinched by a heartless passer-by.
"A lady walked out five metres in front, and hailed a cab and got into it," she told Sky News. "And I went, 'No, that's my cab, I want that cab'."
The building's doorman walked with Polly to the street corner to hail another cab, but it was already too late. With the baby on its way, Polly had no choice but to give birth right there on the pavement. Minutes later baby Ila was born, weighing 7lb 6oz.
"She just came out. I thought I had longer!" McCourt told the New York Daily News.
Luckily, caring passersby rallied round to help, comforting Polly and giving her jumpers and scarves to protect the newborn from the frosty winter which has paralysed parts of the American north-east. Some posted images of the incredible incident to Twitter.
- Angela Ciao (@AngeLa_Ciao) February 24, 2014
One woman even gave Polly the coat off her back, and it is this woman - known only as Isabelle - who Polly wants to locate. She named her new baby daughter Ila Isabelle as a tribute to the kindly stranger.
Polly's husband, Cian, arrived on the scene moments after the birth. He told the New York Daily News he initially feared the worst when he saw the crowd gathered around his wife, but thankfully both mum and baby were safe and well.
Back at home with their three children, the McCourts are now urging for the selfless stranger to come forward. "We want to be able to contact her and say thanks," Polly said.