More than 1,000 people have turned up at the steps of South Australia's Parliament House in support of a proposal to change state abortion laws.

Liberal MP Ben Hood has introduced the private member's bill in the upper house that, if successful, would require people seeking to terminate a pregnancy from 27 weeks and six days to instead deliver their baby alive.

Anti-abortion campaigner and University of Adelaide law professor Joanna Howe said the purpose of the rally was to "talk about the implications" of current abortion laws and "to demand change".

"The question is, as South Australians, do we want her to deliver that child stillborn, or do we expect that child to be delivered alive? That's all this is about," Professor Howe said.

"This is not an anti-woman, anti-abortion piece of legislation."

Currently, the law sets out conditions  for "terminations by medical practitioner after 22 weeks and 6 days".

Mr Hood's amendment proposes to add a new subsection: "A medical practitioner may only intervene to end the pregnancy of a person who is more than 27 weeks and six days pregnant if the intention is to deliver the foetus alive."

Mr Hood said that, under such a scenario, a baby born at 28 weeks would "receive neonatal care" and the mother could choose to put the baby up for adoption.

Rally attendee Bethany Marsh said current abortion laws are "barbaric".

"I think they're sending a really loud, strong message to Adelaide that this is a barbaric injustice, that this issue has gone on for too long, with too many lies being told to people about what the nature of the abortion today is about," she said.

"Innocent lives are being lost through abortions which just don't need to happen after 22 weeks, and we want that to end."

Liberal upper house MP Michelle Lensink, who helped spearhead SA's laws that decriminalised abortion in 2021, said Mr Hood's bill is trying to "correct a wrong that doesn't exist".

"There are a lot of things in their piece of legislation that I think have not been thought through and I think would horrify a lot of South Australians," she said.

"[Decisions to terminate pregnancy] are not flippant decisions for women or their medical team."

The Royal Australian and New Zealand College of Obstetricians and Gynaecologists (RANZCOG) is also opposed to the push.

SA committee chair Heather Waterfall said extremely few terminations happen after 28 weeks.

She said terminations in the third trimester were typically "only in the most extreme situations".

"They're usually because the physical or mental health of the woman is endangered by the pregnancy, or because there is a significant concern with the foetus — the baby that could be born — that there's something wrong with that baby," she said.

The opposition said it would allow its MPs a conscience vote on the issue while Labor has not yet declared whether it will do the same.