"If it's a legitimate rape, the female body has ways to try to shut that whole thing down," Akin said of a rape victim's chances of becoming pregnant.
He also said he would prefer that punishment for rape be focused on the rapist and not "attacking the child."
Akin was interviewed on KTVI's "The Jaco Report," and also talked about numerous campaign issues, such as voter ID laws, the economy and Medicare. KTVI said the interview was conducted earlier in the week.
Akin spokesman Steve Taylor declined to comment Sunday, saying he had not yet seen the interview. The video has been posted on the station's website.
McCaskill, who is seeking a second term, said in an emailed statement Sunday that she found the comments "offensive."
"It is beyond comprehension that someone can be so ignorant about the emotional and physical trauma brought on by rape," McCaskill said. "The ideas that Todd Akin has expressed about the serious crime of rape and the impact on its victims are offensive."
McCaskill also quickly took to Twitter to decry Akin's comments, saying that "As a woman & former prosecutor who handled 100s of rape cases, I'm stunned by Rep Akin's comments about victims this AM."
This month, Akin won the state's Republican U.S. Senate primary by a comfortable margin of victory. During the primary, Akin enhanced his standing with TV ads in which former Arkansas governor and presidential candidate Mike Huckabee praised him as "a courageous conservative" and "a Bible-based Christian" who "supports traditional marriage" and "defends the unborn."
Akin, a former state lawmaker who first won election to the U.S. House in 2000, also has a long-established base among evangelical Christians, and was endorsed in the primary by more than 100 pastors.
Within hours of Akin's win, McCaskill had cast him as a conservative extremist who would jeopardize seniors' health care and retirement savings while putting college out of reach for all but the rich.
Akin countered by portraying McCaskill one of the most vulnerable Democratic incumbents in the nation as a budget-busting, tax-hiking, big-spending liberal.