A Utah woman faced the possibility of being registered as a sex offender after her stepchildren saw her topless in her own home, Fox News reports.

Tilli Buchanan has challenged Utah's lewdness law arguing that it is unconstitutional for women to be discriminated against for showing their breasts, especially in their own home.

The 28-year-old had been working with her husband in the garage when they both took off their shirts to avoid getting dust on them, which the stepchildren saw.

When the birth mother heard of the incident she alerted welfare officials and the charges were filed.

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The judge sided with prosecutors who argued that lewdness is commonly understood to include women's breasts in US society.

"This whole thing is ridiculous," her lawyer Randy Richards said. "She [or other women] have to worry about their kids seeing them topless? It's ridiculous.

"If we lose this, she's on the sex offender registry with child rapists and things of that nature.

"The magnitude of the penalty on this is enormous."

The stepmother spoke to her defence saying she believed there was a troubling double standard between topless men and women.

She was charged with three counts of lewdness, involving a child, while her husband faced no charges.