Anti-abortion activists have pushed for "personhood" in five separate ballot initiatives since 2008. These amendments would likely restrict abortion access as they give unborn fetuses more rights.

Five times now, those amendments have failed, with voters in North Dakota and Colorado rejecting personhood ballot initiatives on Tuesday night. These amendments have failed even in conservative strongholds like Mississippi, which rejected a personhood amendment in 2012.

These efforts have been criticized as too vague and imperfectly worded — even by those within the pro-life movement. But since 2008, activists have continued to pursue these efforts as a way to assert a deeply held belief that life begins at conception. In the wake of two more defeats, here's a quick guide to the personhood movement: where it started, how it works, and why its measures keep failing.

1) Personhood ballot initiatives aim to expand human rights to unborn fetuses

The personhood movement arguably started in 2008 with Kristi Brown (who then went by her maiden name, Burton). Then a 21-year-old law student and anti-abortion activist in Colorado, Brown wanted to see the pro-life movement push to codify in state law the belief that life begins at conception. She described her initial ballot initiative in an interview with Newsweek as laying the "foundation for voters to direct their legislatures and courts to do everything they can to protect life."

The language of subsequent personhood ballot initiatives have looked different depending on the state or the amendment. But what personhood initiatives have in common is that they aim to ascribe additional rights to a developing human life that's still in utero. The general idea is to change the definition of when a "person" becomes a person and give some of the protections that people get to fetuses in the womb.

2) What did the Colorado and North Dakota initiatives say?

The initiatives in North Dakota and Colorado worked a little differently. North Dakota's Measure 1 would have added a paragraph to the state constitution defining life as beginning at conception. This sentence was the core of the amendment: "the inalienable right to life of every human being at any stage of development must be recognized and protected."

North Dakota Choose Life, the group behind Measure 1, argued that its ballot initiative was not a personhood amendment because "it does not confer legal rights on anyone." Most news organizations, however, described it as a personhood initiative because it appeared to have the intent of increasing the rights of a fetus before birth. And North Dakota Measure 1 was supported by national group Personhood USA.

Colorado's Amendment 67 would have added "unborn human beings" to the definition of "person" and "child" in the state's criminal code. It took a more specific approach than the North Dakota effort, which, as a proposed new clause in the constitution, seemed as if it would have affected all of the state's laws and regulations.

3) The three previous personhood ballot initiatives all failed

Before this year's failed initiative, Colorado voters rejected two other personhood ballot initiatives, once in 2008 and again in 2010. In both years, the measures failed by double-digit margins.

Mississippi voted on a personhood ballot initiative in 2012. It failed, too, with more than 55 percent opposing the amendment. Legislators have introduced personhood bills in Oklahoma and Virginia but neither passed.

4) Are all people on in the pro-life community on board with personhood ballot initiatives?

Much of the support for personhood ballot initiatives comes from activists who think the pro-life movement is moving too slowly and incrementally. Instead of passing small abortion restrictions (such as waiting periods and parental notification laws), it makes more sense to pass something sweeping, they argue: new state laws that codify the belief that an unborn child is a person.

Large pro-life groups like Americans United for Life and the National Right to Life Committee have not endorsed personhood ballot initiatives. Part of this is politics: some worried that the amendments (which opponents call draconian abortion bans) will fail so badly they're not worth the effort, and that they will only prove an embarrassment.

And there are also some policy disagreements about what it would actually mean to give personhood rights to fetuses and whether that could have unintended consequences, such as disallowing certain types of birth control. This was what Colorado's new senator, Cory Gardner, a Republican, was getting at in March when he withdrew his support for Amendment 67.

"This was a bad idea driven by good intentions," Gardner told the Denver Post. "I was not right. I can't support personhood now. I can't support personhood going forward. To do it again would be a mistake."

This year's Colorado amendment — which is a less expansive version of the initiatives that failed in 2008 and 2010 — did break through this divide a bit. One leader of the National Right to Life Committee endorsed the effort, although the group itself has not taken a position.

5) Nobody fully understands what it would mean to give unborn fetuses personhood rights

Because no state has ever granted personhood rights to unborn fetuses, it's really unclear how any specific amendment would work in practice. This was especially true with the North Dakota amendment, which didn't give any particular rights to fetuses but instead required "the right to life of every human being at any stage" to be "recognized and protected."

Supporters of both the Colorado and North Dakota initiatives argued that existing protections would still allow for legal abortion. Roe v. Wade, for example, protects legal, elective abortion during the first trimester of pregnancy. Choose Life North Dakota said that protection supersedes any state laws.

But opponents argued that the amendment was written too broadly and that personhood laws would make abortion illegal. The director of North Dakota's only in vitro fertilization clinic said that he would close his practice if Measure 1 passed. Embryos are sometimes discarded in treatment, and his lawyer warned that the practice could put workers at risk of legal action.

"We are covered for malpractice but criminal charges? We're on our own," Steffen Christiensen, who directs the clinic, told ABC News before Election Day. "Sooner or later, someone would try to make an example of us."