(CNN) -- A federal judge ruled Wednesday that North Dakota's abortion law, considered one of the most restrictive in the nation, is unconstitutional.
The law banned most abortions after six weeks, when a fetal heartbeat can be first detected.
"The North Dakota strict ban on abortions at the time when a 'heartbeat' has been detected -- essentially banning all abortions as early as six weeks of pregnancy -- cannot withstand a constitutional challenge," U.S. District Judge Daniel L. Hovland wrote in his decision.
"A woman's constitutional right to terminate a pregnancy before viability has been recognized by the United States Supreme Court for more than 40 years. The United States Supreme Court has clearly determined the dispositive issue presented in this lawsuit. This court is not free to impose its own view of the law."
The law, thought to be the nation's toughest anti-abortion measure, was signed by the state's governor last year.
"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," Gov. Jack Dalrymple said then.
He called the constitutionality of the law "an open question," saying the Supreme Court has never considered the "precise restriction" of the fetal heartbeat aspect.
The governor was not immediately available to comment on Wednesday's decision.
The Center for Reproductive Rights cheered the ruling.
"The court was correct to call this law exactly what it is: a blatant violation of the constitutional guarantees afforded to all women. But women should not be forced to go to court, year after year in state after state, to protect their constitutional rights.
"We hope today's decision, along with the long line of decisions striking down these attempts to choke off access to safe and legal abortion services in the U.S., sends a strong message to politicians across the country that our rights cannot be legislated away," said Nancy Northup, that group's president and CEO.
The ruling was also praised by the director of the Red River Women's Clinic, the only clinic providing abortions in North Dakota.
"The decision is a sigh of relief for us and the women we serve. It's not a surprise. We fully expected the decision to go our way but it's always nice when it finally happens," said Tammi Kromenaker.
Abortion was legalized in all 50 states in 1973 by the U.S. Supreme Court. Statutory time limits on when abortions can take place, however, vary from state to state.
Under Roe v. Wade, abortions are generally permitted until the fetus is considered viable, or able to live outside the womb. Some states have no time limit, while others allow abortion up to the end of the second trimester, about 27 or 28 weeks into the pregnancy.
CNN's Chelsea J. Carter contributed to this report.