The Utah Court of Appeals says the law's "rigorous protections" for younger minors include protecting them from each other.
The decision leaves the teens in the odd position of each being both a victim and a perpetrator in the same offense.
"The Legislature certainly may act to protect the health and safety of children, and may more vigorously protect those of more tender years," Judge Gregory Orme wrote for a three-member panel of the court, which made its decision "with some reluctance."
The girl's Ogden attorneys, Randall Richards and Dee Smith, are considering an appeal to the Utah Supreme Court.
Richards pointed out that Utah law says minors under age 14 do not have the ability to consent to sexual activity.
"It's a paradox," he said. "How can they be old enough to commit an offense if they're not old enough to consent to it?"
According to the court decision, the girl became pregnant after she and the boy engaged in sex in October 2003.
State authorities filed delinquency petitions in July 2004, alleging that each had committed sexual abuse of a child, a second-degree felony if committed by an adult.
The girl appealed the petition, saying her constitutional right to be treated equally under the law had been violated.
Her motion noted that for juveniles who are 16 and 17, having sex with others in their own age group does not qualify as a crime.
Juveniles who are 14 or 15 and have sex with peers can be charged with unlawful conduct with a minor but the law provides for mitigation when the age difference is less than four years, making the offense a misdemeanor.
For adolescents under 14, though, there are no exceptions or mitigation and they are never considered capable of consenting to sex.
A juvenile court judge, although sympathetic to the situation, denied the motion. The girl then admitted to the offense while preserving her right to appeal to a higher court. The boy did not appeal.
The judge ordered the young mother to write a report about the effect of her actions on herself and her baby, to obey the reasonable requests of her parents, to remain under the supervision of the state Division of Child and Family Services, to refrain from unsupervised contact with the baby's father and to provide a DNA sample.
Despite the light sanctions, the conviction could have lingering effects, according to Richards.
If the girl commits a crime when she is an adult, her juvenile record will make it more likely that she will go to prison and could increase the length of her sentence, he said. And there is the emotional toll of knowing she has a sexual abuse conviction.
But the Court of Appeals said lawmakers can constitutionally give greater protection to younger juveniles.
"The Legislature is well within its rights to come down solidly against sexual activity with children of such tender years - anywhere, anytime, any place, and by anyone. . . . And we cannot say such strict treatment does not also rationally further those purposes by strongly discouraging any sexual conduct involving children."