Antonia Lopez has admitted to killing her newborn daughter by throwing her out of a window, but has escaped any time behind bars.

The 16-year-old girl threw her premature baby out of a second-floor window after secretly giving birth in the US on September 30, 2016, reports Kidspot.

The Nebraska teenager was first charged as an adult - possibly facing a 20-year sentence - but was transferred to a juvenile court where she admitted the crime, the Omaha World Herald reported.

Her case was described as a "panicked reaction" rather than a planned-out crime.

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Police say Antonia was alone in her bedroom when she went into labour and gave birth to the premature baby girl. After throwing the tiny baby out the window, Antonia told her mother what she had done, police said.

Her mother found the baby and called the police at 4.12am to report the baby's death.

The baby was pronounced dead upon arrival at the hospital. She only weighed a tiny two pounds, after being born between 25 to 28 weeks, police said.

Antonia was originally charged as an adult but Douglas County Attorney Don Kleine said his office agreed to move the case to the juvenile court after considering the teenager's lack of criminal record, her mental state and her actions.

"It's a horrible case - terrible. To me, after we looked into it, the way things happened, it didn't seem to be anything you would label as an adult-specific act. It wasn't a planned-out, thought-out process. It was more of a panicked reaction," he said.

Antonia denied knowing she was pregnant before giving birth to the two months premature baby. She told police she woke up in pain, and laid a towel in her bedroom believing she was having her period.

But messages to her boyfriend suggest she did know she was expecting a child.

Antonia Lopez was 16 when she gave birth on her bedroom floor. Photo / Supplied
Antonia Lopez was 16 when she gave birth on her bedroom floor. Photo / Supplied

In a series of texts revealed in court, Antonia wrote, "Babe I'm having contractions in my lower abs every couple seconds. It hurts so much."

"I am in so much pain" and later, "I want it to stop".

Then she added, "it was a girl by the way."

Antonia also wrote on Facebook the same day, "Who can do me a huge favour and has a car?"

The juvenile court ordered the 16-year-old continue school and receive medical and psychiatric care while in foster care.