North Dakota governor Jack Dalrymple has approved a ban on abortions after six weeks, which could make the state the nation's most restrictive in terms of abortion rights.
The Republican governor signed legislation that bans abortions if a foetal heartbeat can be detected. That can happen as early as six weeks into a pregnancy.
He also signed a law that makes it illegal to abort a foetus because of genetic defects such as Down's syndrome or based on gender selection.
Abortion rights advocates have said the state will have to defend the legislation in court and protesters have been massing throughout the day over the bill.
The legislation follows an attempt to close the state's only abortion clinic, the Red River Women's Clinic in Fargo.
Pro-life groups say the legislation challenges the Supreme Court ruling of Roe v Wade, which legalises abortion to the point that a foetus is considered viable - usually 22-24 weeks.
"Although the likelihood of this measure surviving a court challenge remains in question, this bill is nevertheless a legitimate attempt by a state legislature to discover the boundaries of Roe v Wade," Dalrymple said.
The North Dakota legislation does not specify how a heartbeat would be detected but doctors performing an abortion on a foetus with a heartbeat could face being jailed for five years and fined $5,000 (£3,300).
Heartbeats can normally be detected early in pregnancy through a vaginal ultrasound.
Earlier this month, Arkansas passed a law banning abortions after 12 weeks if a heartbeat can be detected through an abdominal ultrasound.
Republican lawmakers in North Dakota have endorsed anti-abortion legislation on a number of occasions this year.
Recently, they put forward legislation that would define life as starting at conception, meaning abortion would be banned completely. It is thought this law will come before voters in November next year.