A US judge ordered a lesbian couple in Utah to give up their foster baby on grounds the child would be better off with a heterosexual family, in a "stunning" decision that sparked outrage.

April Hoagland and Beckie Peirce said they were told during a routine court hearing on Tuesday to hand back the one-year-old girl within a week.

"We love her and she loves us, and we haven't done anything wrong," Peirce told The Salt Lake Tribune.

"And the law, as I understand it, reads that any legally married couple can foster and adopt."

Hoagland said juvenile court judge Scott Johansen based his decision on "research to back that children do better in heterosexual homes."

Johansen did not provide details on the research during the court session in the town of Price, the couple said.

Johansen could not be reached for comment on Thursday but a court spokeswoman confirmed to AFP he had issued a ruling in the case, which is sealed because it concerns a juvenile.

The decision has been met with fierce criticism by gay rights groups.

"We were blown away by it," said Troy Williams, director of Equality Utah.

He said the decision was even more baffling as the baby's biological mother supported the adoption, as did the local child services department.

"Everyone was supporting this and it was a shocking experience for all when the judge said, 'Well, I see research that says heterosexual parents are better'," Williams told AFP.

Utah Governor Gary Herbert, who is opposed to same-sex marriage, also weighed into the controversy, saying he was puzzled by the judge's ruling.

"He may not like the law, but he should follow the law," the governor told reporters.

Hoagland and Peirce were allowed to become foster parents after the US Supreme Court made gay marriage legal across the country this summer.

The two women, who are already raising Peirce's two biological teenage children, said they planned to appeal Johansen's ruling and have hired an attorney.

They said they believe the judge was driven by his religious beliefs.

"He's never been in our home, never spent time with the child in our home or our other children so he doesn't know anything about this," Peirce told the local station KUTV.

'Egregious decision'

The court ruling came on the heels of another setback for same-sex couples in the conservative state.

Last week, the influential Mormon Church, which considers same-sex marriage sinful, declared that children of gay couples cannot be blessed as babies or baptized until the age of 18.

"The decision was stunning," Williams said. "And then this (adoption) case followed, so we feel there is a backlash."

However, he added he was hopeful the adoption case would have a happy ending given the uproar.

"Clearly this decision not only is it egregious, it is unconstitutional and we are hopeful for a swift resolution," he said.

This is not the first time Johansen's decisions have raised eyebrows.

In 2012, he told a woman that he would reduce her 13-year-old's sentence if she cut the girl's ponytail in his courtroom.

The girl was in court for allegedly cutting off several inches of hair from a toddler.

And in 1997 he was officially reprimanded for slapping a 16-year-old boy in court.