Deputy Uintah County Attorney Mark Thomas said he filed the felony charge against Arron N. Harrison, 21, who was arrested Thursday and jailed in lieu of $10,000 bail.
Prosecutors are still weighing possible charges against the 17-year-old girl, who is also in custody. Authorities haven't disclosed her name.
"I haven't decided who's more at fault," Mark Watkins, police chief of Naples, 130 miles east of Salt Lake City, told The Associated Press. "She was just as cold as the assailant - we're not going to treat her as a victim."
Thomas said Harrison is expected to appear in court Tuesday and faces one to 15 years in prison if convicted.
Harrison pummeled the girl's abdomen in an effort to cause a miscarriage, Watkins said.
"Her intention was, in fact, to abort the child - to terminate the pregnancy," he said.
The fetus survived, but doctors won't know for certain what injuries it may have sustained until it is born.
The girl spent a day in a hospital before her arrest and is being held at a juvenile detention center, Watkins said.
Thomas said the beating took place late Tuesday, and that the girl initially told her family she had been mugged by an unknown assailant.
The family called police, who got the girl to speak truthfully about what happened, Watkins said.
He said Harrison was an acquaintance of the girl and is not the father of the child.
Neither Thomas nor Watkins knew if either of the two had a lawyer.