Pro-abortion activists demonstrate in front of the US Supreme Court in Washington, DC - AFP

Iowa is set to become the first US state to ban abortions after six weeks of pregnancy, after lawmakers approved the most restrictive ban in the country.

Republican lawmakers in the state passed the bill banning the procedure after the point that a foetal heartbeat is detected.

The bill must now go to Iowa's top official to write the bill into law.

Kim Reynolds, the Republican governor, has not said if she will sign the bill.

If it comes into effect, the law will ban most terminations after six weeks of pregnancy, a move which women's rights groups say will be the country's most restrictive abortion law.

The "heartbeat" bill, as it has come to be known, requires women to have an ultrasound to screen for a foetal heartbeat before they can receive an abortion.

Critics have argued it will make having an abortion illegal before most women even realise they are pregnant.

Pro-abortion activists demonstrate in front of the US Supreme Court in Washington, DC Credit: AFP

The bill exempts women seeking abortions in cases of rape or incest, or if the pregnancy threatens the woman's life.

The bill follows an increasing move towards restricting abortions across the US ahead of the November midterm elections.

Republican lawmakers are eager to advance pro-life causes ahead of the election race in case they lose their majorities.

Last year, Iowa Republicans successfully banned abortions after 20 weeks of pregnancy, among the shortest limits in the US.

US abortion clinics

Democrats in Iowa have criticised the "political" barriers to receiving aboritons. Beth Wessel-Kroeschell, a Democratic lawmaker, spoke against the legislation in the Iowa House of Representatives on Tuesday.

"All women, regardless of age, income or race, should be able to obtain reproductive health services, including abortion, free from political and economic barriers," she said.

Iowa's Planned Parenthood and the American Civil Liberties Union branches have also spoken out against the bill, arguing that it is unconstitutional.

Elizabeth Nash, of the sexual and reproductive health policy group the Guttmacher Institute, told the BBC this was "the most restrictive abortion ban in the country".