A young migrant was arrested Saturday after she was found in the lobby of a Staten Island hospital covered in blood — with the newborn baby boy she had just delivered left behind in a bathroom trash can, police and family said.

Cops responded to Staten Island University Hospital around 1 a.m. after an NYPD officer who was working a side job there spotted the blood-soaked Lucia Garcia, 21, who is originally from Mexico and immigrated to the US about 10 months ago.

The officers and a member of the hospital’s cleaning crew then heard a baby’s cries, and found the infant, who was taken to a Manhattan hospital in stable condition.

The woman’s father, Ambrosio Garcia, 41, said he had brought her to the hospital Friday night because she wasn’t feeling well but that she insisted she wasn’t pregnant and had been hiding her condition from him.

“She said she had her period when she came out of the bathroom,” Garcia told The Post in Spanish Saturday, as his eyes welled up with tears. 

“She had a lot of blood on her,” he recalled.

“Then police went into the bathroom, and another man to clean it. There was a baby boy in there that they found in the trash can.”

The boy was transferred to NYU Langone in healthy condition.

The mom, who is seeking asylum, was charged with endangering the welfare of a child and reckless endangerment, police said.

When Garcia came out of the bathroom, the police officer thought she needed medical attention because of the blood on her clothes and notified hospital staffers, who began examining her, a police source said.

“They thought she was hemorrhaging,” the source said.

Garcia said police took his daughter away after the baby was found — and he hadn’t talked to her since.

“She’s there all alone,” the dad said.

“I feel so bad. I don’t know if she’s okay. I’m sad because I don’t know what I’m going to do. I don’t have money for a lawyer — I don’t make that much. I’m very sad. I feel bad too. I never thought this would’ve happened.”

His daughter was 32 weeks pregnant, law enforcement sources said.

The dad said he has been in the city for four years and his daughter arrived in July.

Her mother remained in Mexico.

The father and daughter share a bedroom in an apartment on Staten Island and she was washing dishes at a nearby restaurant until it closed a few weeks ago.

Garcia said he asked her if she was pregnant about three months ago.

“You could see it a little bit here,” he said about his daughter, motioning to his stomach.

“She said, ‘No, no, I’m not pregnant.'”

Garcia said he was relieved when hospital workers told him his new grandson was doing well, he said.

 “Somebody came and told me that the baby was living [last night] and he was well,” Garcia said.

“That she had put him in the trash. I would love to have a grandchild — it would be my first grandchild.”