RIO DE JANEIRO (AFP) - Three's a crowd? Not in Brazil, where three women have defied deeply conservative trends in Congress and wider traditional mores by celebrating a polyamorous civil union - and will raise a baby to boot.

The happy trio, who reportedly have shared a bed for years and say they want to raise a child, took an oath of love in early October in the presence of Rio de Janeiro notary public Fernanda de Freitas Leitao.

"This union is not just symbolic," because it defines "how they intend to have children," attorney Leitao said.

The lovers - a businesswoman and a dentist who are both 32, and a 34-year-old office manager - have been together for three years and wish to remain anonymous.

Despite salacious media speculation about their supposed love life, they are in fact shy, their lawyer said.

The union is not a formal marriage, because under Brazilian law that would be bigamy.

Neither are they automatically allowed to declare joint income or join a healthcare plan for spouses.

But the civil union is still a big step, according to the lawyer.

"If they seek these rights before a court, they could obtain them - and I think they will," Mr Leitao said.

They also have a better chance now of making good on their plan to create a three-parent family, Mr Leitao said.

"Our union is the fruit of love," the unnamed businesswoman in the trio told the daily O Globo.

"We are preparing for my pregnancy... The legalisation is a way for the baby and for us to not end up abandoned and penniless. We want to enjoy the same maternal rights that everyone else has."

While these are the first women to enter a three-way civil union, a similar ceremony was held in 2012 for a man and two women in Sao Paulo state.

Both arrangements are based on a Supreme Court ruling that in 2011 authorised notary publics to hold civil union ceremonies for gay and lesbian couples.

According to Mr Leitao, "all the principles and fundamentals" of that ruling "can also be applied to polyamorous relationships".

Brazil, the world's most populous Catholic country with a growing Evangelical population, is full of contradictions, including a permissive view of sexual relations typified by the famous tiny Brazilian bikini.

In Congress, one of the most socially conservative in Brazil's history, legislators are currently debating a measure that defines a "family" as the union between a man and a woman.

But at the same time, polyamorous relationships are common in popular culture, including in two popular telenovelas and a TV documentary series.

Fans of the topic will dip into a 1996 novel by Jorge Amado called "Dona Flor And Her Two Husbands".

Anthropologist Antonio Cerdeira Pilao, an expert on polyamory at the Federal University of Rio de Janeiro, says Brazil has had a lax attitude to rules about traditional couples since colonial times, when sexual relations between slave owners and slaves was common.

Not everyone is excited with the racy new model family, though.

"We are on the path towards chaos," fumed Mr Euder Faber Guedes, head of a major evangelical organisation. He considers such relationships an "aberration... opposed to nature as established by God".

"Men with men do not produce children," said lawmaker and evangelical pastor Hidekazu Takayama in a late September congressional debate.

"Satan is laughing, shaking up family structures while arguing for the human rights of modern women," Mr Takayama fulminated on his Facebook page ahead of the debate.