The legislation needs only a final vote in the House, which already passed an earlier version, for Missouri to join a growing group of Midwestern and Southern states attempting to sharply limit abortions.
Republican Gov. Mike Parson is likely to sign the bill. He rallied with supporters of it Wednesday, declaring: "It's a God-given right to live ... and that's why it's important for this legislation to get done."
Missouri's anti-abortion bill includes exceptions for medical emergencies but not for pregnancies caused by rape or incest. Doctors would face five to 15 years in prison for violating the eight-week cutoff. Women who receive abortions wouldn't be prosecuted.
Republican bill handler Sen. Andrew Koenig described it on Thursday as "one of the strongest" abortion bills yet passed in the U.S.
Outnumbered Senate Democratic lawmakers had launched into an attack on the bill before Republican supporters even brought it up for debate.
"So much of this bill is just shaming women into some kind of complacency that says we are vessels of pregnancy rather than understanding that women's lives all hold different stories," St. Louis-area Democratic Sen. Jill Schupp said.
The Missouri legislation comes after Alabama's governor signed a bill Wednesday making performing an abortion a felony in nearly all cases.
Supporters say the Alabama bill is intentionally designed to conflict with the 1973 Roe v. Wade decision that legalized abortion nationally in hopes of sparking a court case that might prompt the justices to revisit abortion rights.
Kentucky, Mississippi, Ohio and Georgia also have approved bans on abortion once fetal cardiac activity can be detected, which can occur in about the sixth week of pregnancy. Some of those laws already have been challenged in court, and similar restrictions in North Dakota and Iowa previously were struck down by judges.
Missouri's bill also includes an outright ban on abortions except in cases of medical emergencies. But unlike Alabama's, it would kick in only if Roe v. Wade is overturned.
If courts don't allow Missouri's proposed eight-week ban to take effect, the bill includes a ladder of less-restrictive time limits that would prohibit abortions at 14, 18 or 20 weeks or pregnancy.
In its Roe v. Wade decision, the Supreme Court noted that viability typically was 24 to 28 weeks. Studies since then have found that some babies born as soon as 22 weeks have survived.
"This is not a piece of legislation that is designed for a challenge," Missouri's Republican House Speaker Elijah Haahr said. "This is the type of legislation that is designed to withstand a challenge and to actually save lives in our state."
A total of 3,903 abortions occurred in Missouri in 2017, the last full year for which the state Department of Health and Senior Services has statistics online. Of those, 1,673 occurred at under 9 weeks and 119 occurred at 20 weeks or later in a pregnancy.
Republicans and Democrats worked for hours to try to reach a compromise on the bill, which included an expansion of tax credits for donations to pregnancy resource centers.
The approved version of the wide-ranging bill bans abortions based solely on race, sex or a diagnosis indicating the potential of Down Syndrome.
It also requires a parent or guardian giving written consent for a minor to get an abortion to first notify the other parent, except if the other parent has been convicted of a violent or sexual crime, is subject to a protection order or is "habitually in an intoxicated or drugged condition." A change was made after hours of late-night negotiations to also remove the requirement when the other parent lacks legal or physical custody.
Still, some lawmakers on both sides of the debate walked away unhappy.
Democrat Schupp said even after changes, it's "an extreme and egregious piece of legislation that puts women's health at risk."
"It is outrageous that it has no exemptions for victims of human trafficking, rape or incest," she said.
Republican Sen. Bob Onder said negotiators went too far to compromise, leaving the bill "a shadow of what it once was."
"This should be entitled not the 'Missouri Stands for the Unborn Act,'" Onder told colleagues on the Senate floor, "but the 'Missouri Stands for the Unborn Act, sort of kind of only after the minority party and the strongest Planned Parenthood lawyers in the country were done with the bill.'"
Associated Press writer David A. Lieb contributed to this report.
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