Gov. Bill Haslam signed an abortion measure Friday that will further limit the few abortions performed in Tennessee past the point of fetal viability — and potentially send doctors to jail if they fail to prove in court that an abortion of a viable fetus was necessary to save a woman's life or prevent substantial or irreversible harm to a "major bodily function of a pregnant woman."

Tennessee becomes one of at least 21 states that explicitly ban abortions beyond viability, but the measure, called the Tennessee Infants Protection Act, goes further than most other bans and could become the subject of a lengthy court challenge.

Tennessee Attorney General Herbert Slatery in early April called the measure "constitutionally suspect" by placing a doctor acting in good faith at risk for felony prosecution.

Slatery also questioned whether a lack of an exception for a woman's emotional or mental health could survive a legal challenge.

Slatery later pledged to defend the bill after it was amended to require two doctors' opinions on fetal viability — but the final measure does not contain any exceptions for mental health risks to the pregnant woman.

“The Tennessee Attorney General has said he would defend this law, and the United States Supreme Court has not yet decided the mental health exception issue discussed in the Attorney General’s opinion," a statement from Haslam said. "For those reasons, I have signed this legislation into law.

“The Tennessee Infants Protection Act prohibits purposely performing post-viability abortions, except when a physician determines in his or her good faith medical judgment that either the unborn child is not viable or that the procedure is necessary to prevent serious risk to the mother," the statement said. "Rather than being a '20-week abortion ban,' as some have described it, the bill requires physicians to assess viability beginning at 20 weeks gestational age, absent a medical emergency."

Doctors groups have opposed the measure, with the American College of Obstetrics and Gynecology noting there is no definitive testing for viability.

The new law comes as Tennessee remains embroiled in two long-running legal challenges to abortion policy.

A 2015 challenge to a 48-hour waiting period required by women seeking an abortion continues in federal court.

That lawsuit filed by the operators of two abortion clinics also challenged a requirement that the state's abortion clinics meet the standards of hospital-like ambulatory surgical treatment centers and a 2012 law requiring doctors who perform abortions to obtain admitting privileges at a local hospital. State lawyers agreed in April to stop enforcing those two measures after the U.S. Supreme Court ruled similar Texas laws to be unconstitutional.

Yet another legal challenge involving Tennessee abortion policy heads to the 6th Circuit Court of Appeals. That challenge involves a dispute on how votes were counted for the 2014 voter-approved Amendment 1 ballot measure, which stripped the right to an abortion from the Tennessee Constitution. If successful, that challenge could invalidate all abortion laws enacted by lawmakers in the past three years.

The new abortion measure signed into law by the governor requires doctors performing an abortion to test for fetal viability after 20 weeks of pregnancy — and to get a second opinion about viability from another physician.

Fetuses aren't thought to be viable before 24 weeks, and physician groups say there is no standard test for viability.

The law also includes only narrow exceptions to the abortion-after-viability rule: a woman's life or substantial or irreversible damage to vital organs.

Efforts by state Democrats to include exceptions for rape or incest failed.

And the law requires doctors performing abortions after 20 weeks to be able to affirmatively prove in court that the fetus was not viable and that the mother's life or physical health was in substantial and irreversible jeopardy.

Doctors found to have violated the law could be subject to up to three years in prison and have their licenses revoked.

Prior state law allowed abortions past the point a fetus could live outside the womb if the woman's health or life was at risk.

No abortion clinics operating in Tennessee provide abortions past 16 weeks.

Department of Health abortion data show that less than one-tenth of a percent of all abortions performed in Tennessee occur between 17 and 20 weeks, but the state does not track abortions at 20 weeks or later. About 1.4 percent of abortions are reported as occurring at unknown points in pregnancy, and state officials say it is possible that some include post-20-week abortions. Those would have to be performed in hospitals.

Reach Anita Wadhwani at or 615-259-8092 and on Twitter @AnitaWadhwani.