The child was found by a dog walker at around 7.10am on Wednesday morning in Roch Valley Woods, a local beauty spot in Heywood, Greater Manchester.
A post-mortem examination was inconclusive and more tests are being carried out to determine the cause of death.
Detective Constable Kelly Bragg, of Greater Manchester Police, confirmed on Thursday that the child was a baby girl.
She added: "Sadly the baby had died and our main priority at this time is the well-being of the baby girl's mother as she may be in need of urgent medical attention.
"If you are the baby's mother I would ask you to get in touch, either directly to me or ask a family member or friend you can trust to contact me on your behalf, and we will do everything we can to help you.
"We also understand that it may be difficult for the baby's mother to come forward and as such I am also asking for anyone who may have information that might assist us to get in touch on 07387 705768."
The area where the child was found was immediately cordoned off on Wednesday morning, with police forensics and sniffer dogs drafted in.
It is in an area of woodland with the River Roch running through it and is popular with local families, dog walkers and children.
Dozens of teddy bears, soft toys and flowers have been laid at the scene by locals.
Millie Crosdale, 18, who laid flowers at the scene on Wednesday, said: "It is so close to home, you don't expect it to happen here. It is heartbreaking really, all you can do is come and pay your respects."
Her friend, Jess Hawkins, 18, said: "This is where we all come down in the summer and it's nice, when it's snowing you bring your kids down with the sleds. You wouldn't think something like that would happen here."