A lesbian woman who gave birth to a child conceived from a fertilized egg provided by her companion can argue for parental rights in the wake of the couple's breakup, the Nevada Supreme Court ruled Thursday.

The unanimous decision overturns a lower court ruling and allows Sha'Kayla St. Mary to seek joint custody of the child born in June 2008 while she was with her former partner, Veronica Lynn Damon.

Damon provided a fertilized egg, with sperm donated by an anonymous donor. The egg was then implanted in St. Mary, who carried the child to term and gave birth in June 2008.

The birth certificate listed St. Mary as the mother.

After the couple broke up a year later, St. Mary signed an affidavit declaring Damon as the child's biological mother, according to court records.

But in seeking custody, visitation and child support, justices said a Clark County judge erred by determining St. Mary was a "mere surrogate" and by not considering a joint-parenting agreement the couple had signed before the child was born.

In 2011, the judge said St. Mary was entitled to third-party visitation but not custody and noted that previous orders determined she had no biological or legal rights under Nevada law.

Justices disagreed and said the joint-parenting agreement was more than a surrogacy arrangement.

"This case presents a situation where two women proffered evidence that could establish or generate a conclusive presumption of maternity to either woman," Justice Nancy Saitta wrote for the court.

"St. Mary asserts that she is a legal mother of the child in addition to Damon, not instead of Damon," the opinion said. "This claim must be given consideration under the Nevada Parentage Act, which does not preclude the child from having two legal mothers."

It added, "When a child has the opportunity to be supported by two loving and fit parents pursuant to a co-parenting agreement, this opportunity is to be given due consideration and must not be foreclosed on account of the parents being of the same sex."

Justices sent the case back to district court for further proceedings.