(CNN) Time and again, when it's introduced in a state legislature, the bill is touted as the most restrictive in the nation. It's often referred to as a "heartbeat bill" and seeks to ban abortions at the time when a fetus' heartbeat can be detected, which can be as early as six weeks into a pregnancy -- before many women even know that they are pregnant.

But just as often as they are introduced, these bills get stymied. They are held up in committees, rejected in legislative votes, vetoed by governors and struck down in courts. Not one state has managed to put a heartbeat bill into lasting practice.

This year, lawmakers in 15 states have lined up to introduce fetal heartbeat legislation. Most of these bills have either died or stalled, pending further action. Georgia was the latest of four states to sign such a bill into law ; the others were Kentucky, Mississippi and Ohio.

Not included in this tally is Alabama, which went further than these other states, enacting a near-total ban on abortion on May 15.

That decision happened to come down on the 46th anniversary of Roe v. Wade, the landmark Supreme Court decision that legalized abortion in the United States.

So what is going on? If the bills flatline over and over again, why do lawmakers keep revitalizing them?

The history of the heartbeat bill

To understand how we got where we are, let's turn back the clock and look at the heartbeat bill's history.

The first bill of this kind was introduced in Ohio in 2011, at the urging of a group called Faith2Action , which describes itself on its website as "the pro-active launching pad for the pro-family movement."

Though it didn't get signed into law, the Ohio bill inspired anti-abortion legislators in states across the country to float their own fetal heartbeat bills. Dozens of similar bills have been introduced in the years since.

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The first state to succeed in enacting a heartbeat bill was North Dakota in 2013, said Elizabeth Nash, the senior state issues manager at the Guttmacher Institute , a research and policy group focused on reproductive health and rights.

The North Dakota bill was challenged through federal courts, where it was struck down at the appellate level, Nash said, before being appealed all the way to the US Supreme Court. The nation's high court declined to take the case, upholding the lower court decision that blocked the bill.

Also in 2013, Arkansas passed a similarly stringent law, though it banned abortions not at six weeks but at 12 weeks, when abdominal ultrasounds generally detect heartbeats. That bill also got struck down and wound its way through the courts, and once again, the US Supreme Court refused to take the case.

Iowa was the next state to enact a heartbeat bill, in May 2018, Nash said. It was challenged in the courts.

The fate of the 2019 laws in Georgia, Mississippi and Ohio will be fought in courts before they go into effect. The Kentucky law has already been blocked by a federal judge.

Why these bills don't make it

What exactly makes a heartbeat bill unconstitutional?

"The US Supreme Court essentially has held" in Roe and subsequent cases "that a state cannot ban abortion before viability," Nash explained. "Viability is determined individually but on average is between about 24 and 28 weeks of pregnancy."

That explains why heartbeat bills, and bills seeking bans at 15 weeks or 20 weeks, are often challenged in courts and knocked down.

Even Steven Aden, chief legal officer and general counsel for the national anti-abortion group Americans United for Life , acknowledged why these kinds of bills don't hold up.

"With all the respect I can muster to my many friends in the heartbeat movement, no heartbeat bill anywhere has ever saved a human life because, to my knowledge, they've all been struck down by federal and state judges -- and that was predictable," Aden said. "They are unconstitutional under current federal constitutional law. They were designed as a vehicle to challenge Roe in the Supreme Court, but they won't get to the Supreme Court unless you can convince four members of the court that a fifth member would go with them to uphold the heartbeat bill."

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It's not that Aden wouldn't love to see Roe v. Wade overturned. He'd like to see the Supreme Court throw it "on the trash heap of history," he said, or give it "a push into the grave."

He's just not convinced that the high court is ready to do that.

Former Ohio Gov. John Kasich, who signed more than 20 laws restricting abortion access during his eight years in office, understood this when he vetoed a heartbeat bill in December, just as he did in 2016.

Kasich cautioned that such bills had been ruled unconstitutional in two other states and warned, in his 2016 veto message , that Ohio would be the "losing party" in a lawsuit and "forced to pay hundreds of thousands of taxpayer dollars to cover the legal fees for the pro-choice activists."

Ban vs. incremental approach

When Kasich chose to veto Ohio's heartbeat bill, he signed into law another bill that bans the most common abortion method used in the second trimester of pregnancy. That law prohibits the dilation and evacuation (D&E) procedure, in which the cervix is dilated and the contents of the uterus extracted.

This is significant because when Kasich faced the choice to sign two potential anti-abortion bills in December, Ohio Right to Life was exclusively behind the D&E ban, or what it calls the "dismemberment abortion ban," and not the heartbeat bill.

That fell in line with how the original heartbeat bill was received in 2011, Nash said. Back then, she said, the heartbeat bill was seen as too "radical" to most abortion opponents.

Instituting incremental restrictions to abortion access rather than outright bans has long been the favored -- and more successful -- approach for fighting abortion in this country, Nash explained. Those piled-on restrictions, she said, have included clinic regulations, waiting periods, insurance coverage bans, mandatory counseling and involuntary ultrasounds -- efforts that have hampered access and, in many cases, closed clinics.

But Ohio Right to Life changed course, saying it would "embrace the heartbeat bill as the next incremental approach to end abortion in Ohio," according to a statement from late last year

"The Supreme Court, as I'm sure you know, is tilting more originalist and conservative," Jamieson Gordon, a spokeswoman for Ohio Right to Life, wrote in an email. "We see this as a favorable time to push legislation that might eventually make it to the Supreme Court, as the ultimate goal is to get Roe v. Wade overturned."

In April, Ohio Gov. Mike DeWine made good on a promise when he signed the heartbeat bill once it landed on his desk.

Why the bills will beat on

Repeated attempts to pass heartbeat bills across the country may be an effort to see what will stick. If circuit courts split on their rulings, Nash said, that may prod the Supreme Court to take a case.

She also speculated that with so many abortion restrictions in place in their states, lawmakers may see the heartbeat bill as the "new trend." And when politicians see counterparts getting attention and earning political capital for their heartbeat bills, it can become a matter of "keeping up with the Joneses and jockeying for position."

Aden, of Americans United for Life, said these "early gestational bills" crop up in conservative states because "there's a feeling that they keep having to push the envelope go where no state has gone before."

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And even though these heartbeat bills will probably continue to get struck down, Aden sees their value as an educational tool and a way to communicate values.

Every time these bills get introduced, "they teach everyone that there is a beating heart at very early stages [of pregnancy], and that indicates life," Aden said.

As long as lawmakers remain "earnest to show their commitment to life," he suggested, these heartbeat bills -- and the legal challenges that follow them -- are destined to go on.