A measure signed by Gov. Jan Brewer will bar most abortions in Arizona after 20 weeks of pregnancy, a ban supporters say protects both mothers and fetuses but one that abortion-rights advocates say is among the most restrictive in the nation.

It bans all abortions after 20 weeks except in a "medical emergency" where an abortion would prevent the mother's death or "substantial and irreversible impairment of a major bodily function."

Seven states have similar restrictions.

The bill signed into law Thursday makes other changes to abortion regulations, including the requirement of an ultrasound 24 hours before the procedure. The law becomes effective 90 days after the Legislature ends its session, which is likely to occur later this month.

Current law allows abortions up until the point of viability, when a fetus could reasonably survive on its own outside the womb. That's considered by many medical experts and abortion clinics to be from 22 to 24 weeks. Current law also allows abortions beyond that to protect the "life or health of the woman" but doesn't define health.

According to the Center for Arizona Policy, the conservative advocacy group behind the bill, the new restrictions will counter the health risks presented by later-term abortions.

About 200 Arizona women who were more than 20 weeks pregnant got an abortion in 2011, according to the group, a number that tracks with prior years of state data. That's about 2percent of the approximately 11,000 abortions a year in the state.

"This bill is intended to protect women," said Center for Arizona Policy President Cathi Herrod. "The safety risks to the mother and the pain endured by the child after 20 weeks is just too high."

Planned Parenthood of Arizona lobbyist Michelle Steinberg called the law the country's "most extreme piece of anti-abortion legislation."

She said the law defines pregnancy in a way that bans abortion two weeks before the other seven states with similar laws, because it calculates gestational age starting with the first day of the last menstrual period rather than the date of conception.

During the hearings on the bill, doctors said many women don't discover their fetus has a severe or life-threatening problem until an ultrasound at about the 20th week. The doctors -- and several women who had faced this issue -- testified that this law would arbitrarily cut off the right for these women to have an abortion.

"My heart goes out to the families that will be impacted," Steinberg said. "Women are being forced to carry children that they know will end up dying within hours of birth."

Debate on the bill was emotional.

Rep. Kimberly Yee, R-Phoenix, who sponsored the bill, said the goal was to protect both the health of women and that of the fetus.

"The state has a compelling interest to protect women from the serious health and safety risks of abortion," Yee said.

Brewer in a news release said the new law is consistent with her support of anti-abortion measures.

"Knowing that abortions become riskier the later they are performed in pregnancy, it only makes sense to prohibit these procedures past twenty weeks," Brewer said in her release.

Sen. Linda Lopez, D-Tucson, said the new law was "a horrible thing to do to women."

"Once again, (Brewer) and the Republicans in the Legislature have decided that they know better than women," she said. "They are again saying that women are incapable of making those decisions."

In addition to banning abortions after 20 weeks, the new law allows doctors to prescribe abortion pills only through the seventh week of pregnancy instead of through the ninth and requires clinics to perform an ultrasound 24 hours before an abortion instead of the current requirement of an hour before.

It also requires abortion clinics to post signs saying it is against the law to coerce a woman into having an abortion; requires physicians to provide additional information about health risks; and requires the state to create a website with abortion health risks, information on adoption agencies and photos or drawings of developing fetuses.

Herrod said the additional provisions are to assure that women have all the information they need when considering an abortion.

Steinberg disagreed.

"These are all sorts of things to make it as difficult as possible for women to access care," she said.

Planned Parenthood has filed lawsuits over the years to stop new abortion restrictions from going into effect, with mixed success. Steinberg said they are still looking at their options with this law.