He added: "It's not the baby's fault for the sin of the father or of the mother."
A King spokesman didn't immediately respond to a request for comment.
Condemnation was swift from both the left and right side of politics.
The nine-term Republican congressman, who represents a sprawling, largely rural 39-county district, has been criticised repeatedly for comments he's made over the years, especially on issues related to race and immigration.
Shortly before the November 2018 election, The Washington Post reported that Mr King met in Austria with the far-right Freedom Party, a group with Nazi ties. Mr King said the meeting was with business leaders, including one person from the Freedom Party, but the newspaper stood by its story.
Mr King also said on Twitter that "culture and demographics are our destiny" and that we "can't restore our civilisation with somebody else's babies".
Soon after the election, Mr King was quoted in a New York Times story saying, "White nationalist, white supremacist, Western civilisation — how did that language become offensive?"
The comments were denounced as racist and led the House to vote 424-1 to rebuke Mr King. Republican leaders also stripped him of his committee assignments.
In a tweet, US Representative Liz Cheney, the No. 3 Republican in House leadership, called Mr King's comments "appalling and bizarre" and added "it's time for him to go".
Several Democratic presidential candidates noted Mr King's comments and urged people to contribute to Democrat J.D. Scholten's campaign.
"You would think it would be pretty easy to come out against rape and incest," one of the presidential candidates, Pete Buttigieg, said in a statement.
"Then again, you'd think it'd be pretty easy to come out against white nationalism."
Meanwhile Democratic frontrunner Kamala Harris slammed Mr King's comments along with the Trump administration official who wants to change the inscription on the Statue of Liberty.
"These guys are just out of their minds! What is that?" Ms Harris said.
"You got one saying, 'Yeah, the Statue of Liberty only applies to people from Europe', and you've got another one saying rape and incest is OK. What is going on with these people? "They do not understand the importance and the responsibility of their jobs."
Although Mr King has usually breezed to victories in the conservative 4th Congressional District, he narrowly won his last election over Mr Scholten.
This year, several candidates have said they will challenge Mr King for the Republican nomination, including conservative state Senator Randy Feenstra.
Mr Scholten also recently announced he'd again run for the seat.
After Mr King's comment on Wednesday, Mr Feenstra said in a statement, "I am 100% pro- life but Steve King's bizarre comments and behaviour diminish our message & damage our cause."
"Yet again, Steve King puts his selfish, hateful ideology above the needs of the people of Iowa's 4th District. Excusing violence — in any way — is entirely unacceptable."
- With AP
