A three-judge panel of the 9th U.S. Circuit Court of Appeals in San Francisco said the law conflicts with federal decisions extending back to Roe vs. Wade.
"Arizona may not deprive a woman of the choice to terminate her pregnancy at any point prior to viability," the court said in a summary of the case.
Fetuses are not considered viable until the 23rd or 24th week of gestation.
"The challenged Arizona statute's medical emergency exception does not transform the law from a prohibition on abortion into a regulation of abortion procedure," the decision said. "Allowing a physician to decide if abortion is medically necessary is not the same as allowing a woman to decide whether to carry her own pregnancy to term. Moreover, regulations involve limitations as to the mode and manner of abortion, not preclusion of the choice to terminate a pregnancy altogether."
The appellate decision overturns a lower court ruling.
Biologists detail four new deep-sea 'killer sponges'
Pistorius testifies he didn't consciously pull trigger when he shot girlfriend