Pregnancy support groups are warning expectant mothers about the dangers of posting photos of their bellies online in case they are stolen for use on porn sites.

Groups in Australia, New Zealand and Canada said people posing as pregnant women had solicited photos for use on pregnancy fetish sites.

The Australian Multiple Birth Association said pregnant women who shared photos with other expectant mothers online might have also unwittingly shared them with people collecting content for sexual, pregnancy fetish websites.

"People are posing as parents or expectant mums of twins or more and joining Facebook groups and sharing a photo of their pregnant belly and asking others to share theirs," Ali Mountfield from the Australian Multiple Birth Association (AMBA) said.

Ms Mountfield said the photos were then used by people who were interested in preggophilia.

"They were scammers or fake profiles and they were then stealing these people's photos for use on websites and this is where the concept, that we have never heard of, called preggophilia has been understood by us," she said.

Ms Mountfield said the Canadian and New Zealand multiple birth associations initially discovered the fake profiles and warned other groups around the world.

She said a person going by the name Afina Petrescu and other pseudonyms infiltrated Australian social media groups.

"I administer one of the Facebook groups for my local AMBA club and that person was in our group and had done that side of things, so what we then were given were examples of where some of these photos were being used on websites that I wouldn't visit normally," she said.

Some preggophilia websites offer sexual content, while some claim to be for admirers of pregnant women and offer "non-nude" photos.

Theft of photos for porn form of sexual abuse: doctor

Doctor Wendell Rosevear works with sexual abuse victims and perpetrators.

"There are as many sexual diversities and appreciations and fantasies as there are people in the world and that's not an issue, unless one person doesn't respect another person's right to choice or privacy," he said.

Dr Rosevear said stealing photos was a form of sexual abuse.

"Essential to valuing people is to respect their choice, so when you go outside respecting choice, it's called abuse," he said.

"But equally, the people who put the information there need to reassess their own choice about whether they put the information there."

Ms Mountfield said the AMBA was now advising women to be careful about posting their photos.

She said some women were shocked their pictures may have been used.

"Some people said, 'Oh, does it really matter?' and others were like, 'Oh, no that's terrible, I'm deleting my photos straight away, but how do you know whether it's been saved offline already?' and so on," she said.

"So the reactions have been quite diverse, but general concern is that beware what you post online."

The AMBA said it and other groups were actively removing people who may be fake members.