SALMON, Idaho (Reuters) - A federal judge on Wednesday struck down a 2011 Idaho law that banned most abortions after 20 weeks of pregnancy, in a decision believed to mark the first time a court has ruled that such a measure was unconstitutional. Arkansas became the latest to enact adopt a "fetal pain" abortion statute last Thursday when the state Senate voted to join the House of Representatives in overriding a gubernatorial veto of the measure. On Wednesday, the Republican-controlled Arkansas Legislature overrode a separate veto by Democratic Governor Mike Beebe to enact the most stringent abortion restrictions in the United States - banning the procedure after 12 weeks of pregnancy. Idaho's measure made it a felony to perform an abortion after 20 weeks unless there was proof the pregnancy endangered the life of the mother. Wednesday's decision by U.S. District Judge B. Lynn Winmill stemmed from one of the first federal lawsuits challenging such statutes, and it was believed to mark the first time a court has struck down a fetal-pain abortion ban on its merits. Last August, the 9th U.S. Circuit Court of Appeals blocked a late-term abortion ban in Arizona from being enforced and agreed to an expedited review of that measure in a case that was argued before a three-judge panel in November. The Idaho lawsuit was brought in September 2011 by then 33-year-old Jennie Linn McCormack, who according to prosecutors had terminated her own pregnancy at 20 to 21 weeks using abortion pills she obtained from an online distributor. On Wednesday, Winmill sided with Hearn and McCormack, ruling the statutes in question infringed on a woman's constitutional right to terminate her pregnancy under the 1973 U.S. Supreme Court decision known as Roe v. Wade. That landmark opinion legalized abortion while allowing states to place restrictions on the procedure from the time a fetus could viably survive outside the womb, except in cases in which a woman's health was otherwise at risk. A full-term pregnancy typically is about 40 weeks. Referring to Idaho's 20-week abortion ban, Winmill wrote, "Here, an outright ban on abortions at or after 20 weeks' gestation places, not just a substantial obstacle, but an absolute obstacle, in the path of women seeking such abortions." Winmill also chided the Idaho Legislature for enacting the 20-week abortion ban in the face of an opinion by the Idaho attorney general that strongly suggested the law was unconstitutional under existing court precedents. Sponsors of the measure had argued that such a restriction was needed in Idaho, although state figures showed that about 1 percent of abortions performed there involved pregnancies at 20 weeks or more. In another portion of his ruling, Winmill said Idaho's restriction of late-term abortions to those performed in hospitals was unconstitutional because it could effectively limit the use of so-called abortion pills, widely known as RU486, that allow women to end their own pregnancies at home. The question is not limited to whether the hospital-only requirement substantially burdens a woman's right to choose a medical abortion in her second trimester. Instead, the question is whether the hospital-only requirement substantially burdens a woman's right to choose any type of second-trimester abortion. The Supreme Court has twice answered this question, 'yes,'" Winmill wrote. Hearn told Reuters that Winmill's decision marked the first time a federal court found it unconstitutional to subject physicians to criminal prosecution for prescribing medicines like RU486 beyond label recommendations that they be used for ending pregnancies up to just seven weeks of gestation. Marvin Thomas Richardson, an outspoken anti-abortion activist in Idaho, condemned the court's decision, saying "Federal judges have no business ruling on this," adding, "As soon as sperm meets egg, that's a legal person on Earth." Reporting by Laura Zuckerman; Editing by Steve Gorman, Peter Cooney and Lisa Shumaker