According to people on the scene, the young woman - who is yet to be identified - had already gone into labour when she left her apartment. Her doorman walked her up the street to try and find a cab, but the baby decided it couldn't wait.
One witness said: 'She was like "Oh my god, the baby's coming," and then she described being able to see the child's head coming out.
Just a few seconds later, the little girl was born in front of a crowd of spectators.
By-standers then offered to wrap their scarves around the baby to keep her warm.
'I happened to be crossing the street and the baby came out…all i could do is cover her up with my scarf. Amazing - a miracle on 3rd Avenue.'
The mother and child are now said to be recovering well at a hospital in New York. It was apparently the woman's third child with her husband, who must have been miffed to have missed out on his daughter's birth, when so many others got to see it.