The U.S. Supreme Court will decide later this year whether a corporation can have religious beliefs. Maggie Koerth-Baker looks at the science of birth control, and how it might inform the debate.

Later this year, the US Supreme Court will issue a ruling in the case of Sebelius vs. Hobby Lobby Stores Inc. — answering whether a corporation can have religious beliefs that enable it to opt out of the mandate requiring company-purchased insurance to cover all forms of birth control. That's the legal question. But the case also orbits around a separate question that has roots in both science and religious belief. Hobby Lobby doesn't want to provide health insurance that covers the costs of birth control because that includes IUDs, or intrauterine devices.

The owners of Hobby Lobby believe that IUDs actually cause abortions. Birth control activists say IUDs never cause abortions, and work by preventing pregnancy, just like you'd expect birth control to do. Who is right? According to scientific research, neither of them — though the birth control activists are much closer to being right than Hobby Lobby.

IUDs are basically just little pieces of plastic. In the United States, there are two types available, both shaped like a capital "T". One, the ParaGard, has thin copper wire wrapped around the bars of the T. The other, Mirena, secrets a type of progestin, a hormone that's used in other forms of birth control, like the Pill. Either way, a doctor uses a thin tube to slide the IUD, T arms folded, through a woman's cervix and into her uterus. You can leave it there for years. There's nothing to remember, as with the pill or a condom. And it has a low failure rate — fewer than 1 in 100 women will get pregnant in a year while using an IUD. Compare that to the Pill — 9 pregnant women out of 100 in a year — or a condom — 18 out of 100 — and you can see the appeal. As a bonus, the IUD, and particularly the ParaGard, is also a form of birth control that nursing mothers can use without affecting their milk supply.

But how do they work? To understand this better, I turned both to scientific research — most of which dates to the late 1970s and early 1980s, little has been done since — and to experts. In particular, Horacio Croxatto. He's a surgeon and biologist, and founder of the Chilean Institute of Reproductive Medicine. He's also one of the scientists who conducted the studies that people are referring to when they say that IUDs don't cause abortions.

The confusion surrounding how IUDs work dates back to animal studies conducted in the 1960s, Dr. Croxatto told me. The first study to examine the mechanism of IUDs was done in 1964 on rats and clearly showed that, in rats, IUDs do prevent fertilized eggs from implanting in the uterus. But, Dr. Croxatto said, those results turned out to not be applicable to human women. Or, for that matter, to females of other animal species.

Rats have a reproductive system that's very different from humans. Instead of a large, central uterus, their uterus branches into two distinct horns. Because of that, scientists were able to use one horn as the control and put an IUD in the other. They mated the rats, and then killed and dissected them at 1, 2, 5, and 10 days after mating. When they cut the rats open, they could see how eggs were being fertilized and developing normally on both sides, but those fertilized eggs were only implanting and growing in the uterine horn that had no IUD. In other words, the IUD was preventing fertilized eggs from becoming viable pregnancies. Biologists and doctors define pregnancy as beginning at implantation. So, technically, preventing implantation isn't an abortion. That said, the people who own Hobby Lobby define pregnancy differently, as beginning at fertilization. If humans were like rats, then their fears would be justified.

But, when scientists tried to replicate the results in other test animals, they found completely different mechanisms. For instance, a similar study in sheep, which also have uterine horns, showed that eggs weren't being fertilized in the control side, or the side with an IUD. Researchers eventually figured out that the presence of an IUD made the peristaltic contractions — muscle movements in the uterus that usually move sperm towards the fallopian tubes — operate in reverse.

When scientists tested the mechanism of IUDs in other animals they also found it working in different ways. Basically, Croxatto said, you can't extrapolate the results in one species to assume how IUDs work in another.

The Human Studies

It wasn't until the 1970s that scientists actually studied the mechanism of IUDs in human women. The studies done on animals couldn't be replicated exactly in humans. (Anybody up for mating followed by death and dissection?) So, instead, researchers came up with other ways to do the tests.

For example, they looked for the hormone released when an egg has been fertilized and is in the process of implanting in the uterus. Human chorionic gonadotropin is the same hormone that a pregnancy test looks for and it's present beginning about 7 days after ovulation. HCG can't tell you much about the earliest stages of fertilization, but we know that levels of the hormone rise and then fall in women who have very early miscarriages that failed to implant, so it stands to reason that, if IUDs were preventing fertilized eggs from implanting, you'd see the same sort of thing.

The trouble is, there's lots of different ways to measure HCG. Some of those methods are more sensitive than others. And it's a measurement that's easy to get wrong, with a risk of both false positives and negatives. Because of this, we don't even know, exactly, what percentage of pregnancies end in miscarriage — some studies say it's as low as 6%, some say as high as 57%. It's not surprising, then, that studies of HCG levels in women with IUDs have had varying results. In 2007, though, Dr. Croxatto reviewed the research. He found five studies that showed transient increases in HCG in 15-44% of the cycles of women with IUDs. He found 14 studies that showed HCG increasing in only 0-2.7% of the cycles of women with IUDs. In general, the 14 studies that suggest HCG levels don't increase very often, if at all, in women with IUDs were done with better techniques and are considered more reliable.

With the help of microscopes, scientists also searched for actual eggs, both fertilized and unfertilized. You can do this in a couple of ways. First, scientists can inject liquid into the uterus, flushing it out like you'd flush out hard-to-reach engine systems in a car. Then, you collect the fluid that comes out and look for eggs. For instance, a study that Croxatto did in 1974 found eggs (some fertilized, some not) in 22% of the uterine flushes they did on women who weren't using birth control. In women with IUDs, on the other hand, flushing revealed eggs in only 1.5% of the searches.

Fourteen years later, a study done by a different set of researchers turned up similar results. This study examined the fallopian tubes of women who had gone in for surgical sterilizations. Some of them had IUDs, some didn't. Most of the women abstained from sex before their surgeries, at the request of the researchers. But among the small group that did not, the scientists found fertilized eggs in 50% of the control subjects. They found none in the women with IUDs.

That study also had an interesting twist. Among the women who had abstained from sex, scientists found significantly lower numbers of unfertilized eggs in the women with IUDs. Why would having an IUD reduce the number of unfertilized eggs in a woman's reproductive tract? Croxatto says that seemingly weird result points towards the mechanism that makes IUDs work.

Biological Recoil

All over your body, your immune system is hard at work, looking for invaders and jumping into action to drive them out. The front line shock troops in this battle are white blood cells, which seek out unwelcome microbes and attack them — poisoning some and devouring others in a process called phagocytosis. The bulk of evidence suggests that this same process is what makes IUDs work, Croxatto says. When you put an IUD in your uterus, your immune system registers it as an intruder and starts to attack. White blood cells can't kill a piece of plastic and copper, but they give it their best shot, and those efforts end up killing the majority of sperm that reach the uterus. The effect is even stronger in IUDs made with copper, like ParaGard, because copper ions are also toxic to sperm.

There have been at least two studies that asked women who were planning a sterilization surgery to have sex first, so scientists could look for sperm in their discarded fallopian tubes. As you might expect, women who weren't using birth control had lots of sperm up in there. Women who were using inert plastic IUDs (a type of IUD that's no longer sold in the US) had some sperm. Women using copper IUDs had none. Not a single sperm.

That's a stark contrast between humans and those rats that IUDs were first tested on. Croxatto says the difference lies in how sperm travel. In rats, sperm reach the uterus protected by a bubble of semen that keeps angry white blood cells at bay. Human sperm have no such luck, their semen gets left behind in the vestibule of the vagina. So if an IUD is in place, the immune response it creates kills sperm.

And Croxatto thinks that same process explains why scientists also found fewer unfertilized eggs in women who used IUDs. There's not really a barrier between the uterus and the fallopian tubes in human women. So uterine fluid, filled with white blood cells and copper ions, is free to move into the fallopian tubes, where it destroys unfertilized eggs the same way it destroys sperm. That idea isn't as well established as the effect on sperm. But it makes sense.

Science and religion

So, does that mean the birth control activists are right? That IUDs never prevent fertilized eggs from implanting? Not exactly.

While we know IUDs do an impressive job of preventing fertilization, we also know that they're perfectly capable of preventing implantation, as well. ParaGard, the copper IUD, can be used as emergency contraception. Say you have sex, and the condom breaks. You can go in to the doctor the next day, or even as many as five days later, get a ParaGard, and almost eliminate your chances of getting pregnant. But sperm reach the fallopian tubes within minutes of ejaculation. So the IUD can't be working by killing off sperm. "In that case, it's clearly abortifacient," Croxatto says. "No doubt about that."

When birth control activists and religious groups face off about how IUDs work, what they're actually arguing over is the primary mechanism of IUDs. People like the owners of Hobby Lobby believe that IUDs mostly work by preventing implantation, which is, to them, an abortion. Activists (and scientists) point to the data that strongly suggests the opposite — IUDs can prevent implantation, but they mostly work by preventing eggs from being fertilized in the first place. Then, of course, there's also that pesky semantic disagreement about what constitutes a pregnancy and, thus, an abortion.

This is what I mean by both sides being right, but birth control activists being moreso. If you go by the scientific definition, where pregnancy begins at implantation, then IUDs definitely don't work by causing abortions. If your religious beliefs lead you to think pregnancy begins at fertilization, well, the data suggests that, sometimes, rarely, IUDs used as birth control might abort a fertilized egg. In that way, everybody's right. But the science — what we know about IUDs from evidence — suggests that the primary mechanism is to prevent fertilization, not to prevent implantation. So, in that way, Planned Parenthood is more correct than Hobby Lobby, no matter what deeply held religious beliefs the company's owners may have.

There's more to the research than I've written about here. If you want to learn more about how IUDs work and how we know how they work, I'd recommend these sources:

A 2007 peer-reviewed paper by Horacio Croxatto that reviews and summarizes previously published research.

A 1989 editorial published in the scientific journal Studies in Family Planning by Irving Sivin, senior scientist at The Population Council.

A 1987 World Health Organization report on IUDs, their safety and mechanism of action.

DISCLAIMER: I'm personally pro-choice. I think birth control should be covered as a part of health care, no different than birth, pap smears, or any other aspect of women's health. I use an IUD, myself, and do not particularly care if it occasionally prevents a fertilized egg from implanting in my uterus.

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