A federal appeals court struck down one of the nation’s toughest abortion restrictions on Wednesday, agreeing with a lower court that a state law unconstitutionally burdens women by banning abortions after the 12th week of pregnancy if a doctor can detect a fetal heartbeat. The United States Court of Appeals for the Eighth Circuit sided with doctors who challenged the law, ruling that abortion restrictions must be based on a fetus’s ability to live outside the womb, not the presence of a fetal heartbeat, which can be detected weeks earlier. The court said that standard was established by previous United States Supreme Court rulings. The ruling upholds a decision by a federal judge in Arkansas who struck down the 2013 law before it could take effect, shortly after legislators approved the change. But the federal judge left in place other parts of the law that required doctors to tell women if a fetal heartbeat was present; the appeals court also kept those elements in place.