While the Hobby Lobby and Conestoga Wood challenges to the contraceptive mandate headed to the Supreme Court this week, as well as suits from Catholic non-profits in the lower courts, are widely perceived as being about access to contraception, the reality is that they largely hinge on objections to one specific form of contraception: emergency contraceptive pills (ECPs). This distinction is important because in building the case for their right to refuse to provide contraceptives of all kinds religious conservatives have leveraged small gaps in scientific understanding to claim that it functions as an abortifacient.

Every one of the plaintiffs in the nearly 80 suits filed against the mandate objects to providing Plan B and Ella (both ECPs) because, they claim, they are “abortion-inducing drugs.” They’re even the main point of contention in the suits filed by non-Catholic complainants, including Hobby Lobby, who have no doctrinal opposition to birth control (with the exception of the IUD, which they also claim is an abortifacient).

It’s the claim that employers could be required to pay for abortifacients that gives the suits their moral weight. With nearly all women using birth control at some point, it’s hard to make the case that contraception is so morally objectionable that employers should be allowed to refuse to provide it. But make the claim that the mandate forces employers to cover abortion-inducing drugs and it’s a whole different story. The image of devout, habit-wearing nuns being forced against their conscience to provide “abortion drugs” is what makes the Little Sisters of the Poor case so compelling and what may have won the sisters a stay against the mandate from Justice Sonya Sotomayor.

But ECPs are not abortifacients. They can’t terminate an existing pregnancy, which the American College of OB/GYNs, the Food and Drug Administration and just about any other legitimate medical authority has defined for the last 50 years as the point at which an embryo implants in the uterus. Emergency contraception works mainly by preventing ovulation and sometimes by making it harder for the egg and sperm to meet up.

Religious conservatives’ objection to ECPs is based on their assertion that they may in some cases prevent a newly fertilized embryo from implanting in the womb, which they contend is an abortion based on the religious belief that life begins “at the moment of conception.”

Claims that ECPs are abortifacients were possible to make when the drugs became available in the late 1990s because scientists didn’t understand exactly how they worked. They knew that most of the time they prevented ovulation or fertilization, but didn’t know enough about the mechanism of action to completely discount that possibility that the pills could alter the lining of the uterus enough to prevent implantation. The fact it couldn’t be ruled it out gave religious conservatives who opposed contraception (and were agitating for the right to refuse to provide it in public accommodations like drugstores) an opening to claim that the drugs were potentially abortifacient. They then argued that the same conscience exemptions extended to the provision of abortion ought to apply to ECPs as well. Thus, a tiny amount of uncertainty was leveraged to paint drugs ironically hailed as a breakthrough in the prevention of abortion as abortifacients.

In response, emergency contraception expert James Trussell and a co-author wrote in the Journal of the American Medical Association that these manufactured “politics of doubt” were a “powerful way of subverting sound science-based public policy.”

The Catholic bishops were among the earliest promulgators of the conflation of ECPs and abortion. Around the time the first ECPs were approved, new laws were being considered to require insurers to cover prescription contraceptives, which were routinely excluded from health insurance. The Catholic Church was looking for ways to quash the measures based on its moral opposition to non-procreative sex.

They were initially successful in claiming that insurance shouldn’t pay for contraception because it was a “lifestyle” medication. But the approval and subsequent rapid insurance coverage of Viagra in 1998 nullified that argument, so the bishops’ lobby then argued that insurers would be required to cover abortifacients. When the federal government moved in 2000 to require participating insurers in the federal employees insurance program to cover contraceptives, the U.S. Conference of Catholic Bishops proclaimed that it would “force health plans to cover controversial abortifacients such as the new so-called ‘morning after’ product.”

Fast forward a decade or so and the Catholic bishops were the first to object to the contraceptive mandate in the Affordable Care Act because it encompassed “abortion-inducing drugs,” which soon became gospel among conservatives. Mitt Romney criticized the mandate for requiring coverage of “abortive pills” in a 2012 campaign speech. Plaintiffs in the contraception suits ratcheted up assertions that emergency contraceptives are “known abortifacients” by making the baseless claims that they can “dislodge an already implanted embryo” and “kill embryos even after they have implanted in the uterus.”

But a better scientific understanding of ECPs has undercut even these objections. The latest evidence says that Plan B, which accounted for 99 percent of the market in 2012, doesn’t affect implantation at all—if it fails to prevent ovulation it doesn’t work. In 2010, Health Progress, the Catholic Health Association’s own journal, admitted that Plan B “is not an abortifacient.”

Ella, a newer ECP that can prevent pregnancy for up to five days after uncontracepted sex, doesn’t have post-fertilization effects according to all available evidence. Its ability to prevent pregnancy “can be fully accounted for by mechanisms that do not involve interference” with implantation, according to the latest review of the literatureconducted by Trussell and colleagues.

While studies haven’t been conducted in such a way that can absolutely rule out post-fertilization effects, it’s worth noting that, ironically, Catholic hospitals accept the use of emergency contraception. The Ethical and Religious Directives for Catholic Health Care Services, the bishops’ own guide to what Catholic hospitals can and can’t do, say that a woman who has been raped can be treated with ECPs (with the caveat that there is “no evidence that conception has occurred”). This would be absolutely prohibited if emergency contraception was believed to be an abortifacient.

Last year the German bishops’ conference said that Catholic hospitals in that country could give ECPs to women who had been raped. The Vatican concurred. “To consider the possibility of using a drug whose active ingredient is a contraceptive in the case of a woman who has been raped seems acceptable to me,” said Bishop Ignacio Carrasco de Paula, the president of the Vatican’s Pontifical Academy for Life.

Yet here in the United States, the bishops continue to insist that the evidence about Plan B isn’t conclusive. Richard Doerflinger, the bishops’ chief lobbyist, claimed just last year that there are “warring studies and conflicting viewpoints” on whether Plan B can prevent implantation. Except that there aren’t conflicting studies, just conflicting viewpoints that the bishops helped manufacture and which are now being used to justify the denial of contraception to women.