The measure would triple Missouri's current 24-hour waiting period and put the state in line with Utah and South Dakota as the only states to mandate a 72-hour time frame. Missouri currently has only one clinic performing elective abortions.
The House voted 111-39 in favor of the measure Wednesday, sending it to Nixon, who has previously allowed other abortion restrictions to become law without his signature. That included a measure last year that requires doctors to be in the room for the initial dose of a drug used in medical abortions.
Nixon said Tuesday that he would review the extended waiting period and act in a manner consistent with his other actions on abortion legislation.
Senators passed the waiting period measure earlier this week after Democrats agreed to stop filibustering the bill in exchange for Republicans dropping other measures the Democrats opposed, related to union fees and photo identification requirements for elections.
Under both current law and the new legislation, Missouri's abortion waiting period doesn't apply in instances deemed by a doctor to be a medical emergency. But women do have to wait in cases of rape and incest.
Supporters argue that women need more time to digest information received by a doctor. In addition to the waiting period, Missouri's current abortion law requires doctors to provide women with a variety of written information about the procedure, and they must be given the opportunity to hear the fetus' heartbeat on an ultrasound.
"Life is precious. I want to make sure the unborn child has a chance to survive," Sen. David Sater, a Republican pharmacist from rural southwest Missouri, said during debate on the legislation he sponsored.
Opponents said the longer waiting period would push woman further into pregnancy before an abortion, which can increase risk. They argued it would also make women pay for expensive hotel visits or drive multiple times to a clinic. A Planned Parenthood in St. Louis is the only facility in Missouri that currently performs elective abortions.
"The idea that a woman would not have taken this time already is insulting," said Rep. Genise Montecillo, a St. Louis Democrat.
About 25 abortion-rights advocates gathered at the state Capitol on Monday and stayed throughout the night into Wednesday as part of what they described as a 72-hour "citizen filibuster" against the bill. Speakers took turns talking about the negative impacts of the 72-hour waiting period from a lectern on the Capitol steps. They plan to continue until Thursday afternoon.
"I think the 72 hours is way too long," St. Louis resident Madi Mauck said while taking a break from speaking at the lectern. "Making them wait longer is an emotional and financial burden."
Republican supporters of the measure said the longer waiting period could help reduce abortions and that lawmakers had a moral obligation to pass the bill.
"The objectors stood and focused on the rights of the mother," Rep. Kevin Elmer, a southwest MissouriRepublican, said before Wednesday's final House vote. "I stand here for the rights of the unborn."
Democrats argued the longer waiting period would just force women to go to abortion facilities in neighboring states, citing a clinic in Overland Park, Kansas, and a location in Belleville, Illinois.
State figures show there were about 9,000 abortions performed on Missouri residents in 2012 and that the number of abortions for Missourians has declined each year since 2008.
While Missouri's bill is similar to those Utah and South Dakota law, there are some key differences.
Under South Dakota's law, weekends and holidays don't count toward the 72-hour period. Utah's waiting period does not apply when a woman becomes pregnant because of rape or incest, or when she is younger than 14.
A clause in Missouri's bill if a court finds the three-day time frame unconstitutional would seek to immediately revert to the 24-hour period.
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