A North Dakota law criminalizing abortion when a fetal heartbeat can be detected, normally about the sixth week of pregnancy, is unconstitutional, a federal judge ruled Wednesday.

North Dakota's law — along with a similar Arkansas law struck down last month — were two in a series of laws passed by states last year that imposed new regulations on abortion procedures and those who perform them, sparking state and federal court challenges by rights proponents.

"The state of North Dakota has presented no reliable medical evidence to justify the passage of this troubling law," U.S. District Judge Daniel Hovland in Bismarck said in his ruling.

U.S. Supreme Court precedent cited by Hovland holds that abortion is a constitutional right before a fetus is capable of surviving outside its mother's womb. The state argued that restrictions on the procedure may begin when a heartbeat can be detected. The state also asserted a fetus could be considered viable at the moment of conception.

The Red River Women's Clinic in Fargo said in its lawsuit to overturn the statute that viability typically begins about the 24th week of a pregnancy. The facility is the only abortion clinic in the state.

In Arkansas last month, U.S. District Judge Susan Webber Wright in Little Rock held unconstitutional a law blocking abortions after the 12th week of a pregnancy, except in cases of rape and incest or to save the life of the mother. Arkansas Attorney General Dustin McDaniel said Friday he will appeal Wright's decision.

North Dakota Attorney General Wayne Stenehjem said in a statement that his office and other state officials are reviewing the ruling, which he called "not wholly unexpected," before deciding whether to challenge it on appeal.

In July, Hovland issued an order temporarily blocking the law, passed by the Legislature last year.