U.S. District Judge Joseph H. McKinley Jr. ruled that the 2018 law would create a "substantial obstacle" to a woman's right to an abortion, violating constitutionally protected privacy rights.
Kentucky's only abortion clinic challenged the law right after it was signed by Republican Gov. Matt Bevin. A consent order had suspended its enforcement pending the outcome of last year's trial in which Bevin's legal team and ACLU attorneys argued the case.
The law takes aim at an abortion procedure known as "dilation and evacuation." The procedure was used in 537 of 3,312 abortions in Kentucky in 2016, according to state statistics.
McKinley wrote that standard D&E procedures account for virtually all second-trimester abortions in Kentucky. The law would "unduly burden" women seeking the procedure, he said.
"If the Act goes into effect, standard D&E abortions will no longer be performed in the Commonwealth due to ethical and legal concerns regarding compliance with the law," he wrote.
The result, the judge said, would be that women lose "the right to obtain a pre-viability abortion anywhere in the Commonwealth of Kentucky after 15 weeks."
ACLU attorney Alexa Kolbi-Molinas said the judge's ruling "affirms that health, not politics, will guide important medical decisions about pregnancy."
"Laws like this are part of an orchestrated national strategy by anti-abortion politicians to push abortion out of reach entirely," she said in a statement.
Bevin spokeswoman Elizabeth Goss Kuhn said the governor's legal team will appeal McKinley's decision to the 6th U.S. Circuit Court of Appeals. She predicted the law "will ultimately be upheld."
"We profoundly disagree with the court's decision and will take this case all the way to the Supreme Court, if necessary, to protect unborn children from being dismembered limb by limb while still alive," she said in a statement.
Kentucky is one of many Republican-dominated states seeking to enact restrictions on abortion as conservatives take aim at the 1973 U.S. Supreme Court decision that legalized abortion. Energized by new conservatives on the Supreme Court, abortion opponents in multiple states hope to ignite new legal battles that could prompt the justices to revisit Roe v. Wade.
Steve Pitt, lead attorney for Bevin's legal team, described the second-trimester procedure as "brutal, gruesome and inhumane" during last year's trial over the law in Louisville.
The state's lawyers say the law would still allow use of the D&E procedure, but only after doctors used other methods to induce fetal death. Abortion providers violating the law would be guilty of a felony. Women undergoing such abortions would not face prosecution.
The judge said Friday the law would require women seeking a second-trimester abortion at and after 15 weeks to "endure a medically unnecessary and invasive procedure that may increase the duration of an otherwise one-day standard D&E abortion."
McKinley said the plaintiffs "successfully showed the Act will operate as a substantial obstacle to a woman's right to an abortion before the fetus reached viability — a violation of a woman's Fourteenth Amendment rights to privacy and bodily integrity."
The case is part of a bitter legal fight in Kentucky over abortion policy.
Kentucky Republicans have pushed through a series of measures putting limits and conditions on abortion since assuming complete control of the state's legislature in 2017. Those laws have triggered several legal challenges.