Free speech has been dealt a blow at Adelaide Airport, pro-life advocates say, after an anti-abortion billboard advertisement was rejected without explanation.

Right to Life Association of South Australia proposed an advertisement that used aeroplane images and a pun to draw attention to the state's abortion rate.

To be erected on prominent Tapleys Hill Road just west of the CBD, the billboard's big block letters would have read: "One in five unborn South Australians never get to take off."

But Adelaide Airport Limited rejected it without disclosing its reasons, the advocates say.

"We live in a democracy and sometimes you're entitled to question the status quo and that's what we're trying to do," association president Michael Hall said.

"They have the right to [reject it] but we think there's overriding issues here like public debate on controversial issues."

Mr Hall said the group wanted to inform people that "one in five pregnancies in SA were terminated".

The Right to Life Association of South Australia wanted the message posted on this billboard site outside Adelaide Airport. ( ABC News: Candice Marcus )

"Not everyone in SA is in favour of abortion, so it's a controversial issue and it needs to be discussed," he said.

In a statement provided to the ABC, an airport spokesperson said it was mindful of the impact the message might have on the community.

"Adelaide Airport Ltd, as a private company, assesses all proposed on-airport advertising, which includes balancing the impact it may have on members of the community," the spokesperson said.

Mr Hall conceded the billboard would "probably upset some people" because it highlighted a controversial issue.

"But sometimes you can't avoid upsetting people," he said.

He said the association would continue to try to get its message across and would not let the setback deter its campaign.