A woman has been charged with a crime that could force her to register as a sex offender after her stepchildren allegedly saw her topless inside her own home.

Tilli Buchanan, 28, was working with her husband in their garage at their Utah home in late 2017 or early 2018 when they both removed their shirts to prevent dust from getting on them, The Associated Press reported.

The children, ages nine and 13, saw the couple both topless - and when their birth mother later heard about the incident, she mentioned it to child welfare officials because she was "alarmed", the report read.

The stepmother was charged in February with three counts of misdemeanour lewdness involving a child. The husband faces no charges.

If convicted, the woman could be forced to register as a sex offender for 10 years.

"If we lose this, she's on the sex offender registry with child rapists and things of that nature," Ms Buchanan's lawyer, Randy Richards, told the Salt Lake Tribune. "The magnitude of the penalty on this is enormous."

Ms Buchanan is now fighting the case, pointing to a court ruling that overturned a topless ban in Colorado and helped fuel the "Free the Nipple" movement. She's also spoken out against what she views as a double standard regarding topless men and women.

"It was in the privacy of my own home. My husband was right next to me in the same exact manner that I was, and he's not being prosecuted," she said.

But prosecutors in West Valley City argued Ms Buchanan was "under the influence of alcohol" when she removed her shirt and made a comment that she would put it back on only if her husband exposed himself, the Tribune reported.

Several states around the US have been engaged in debates over whether women should be allowed to be topless in public. The Free the Nipple group has had mixed success in fighting on behalf of women who want to be treated the same as men on the issue.

The judge expects to reach a decision in about two months, the Tribune reported.

Reactions on social media, however, have blasted the court case, calling it "stupidity".

The Associated Press contributed to this story.