Britain is on track to become the first country to permit "three parent" pregnancies, with the first tri-parental babies due to be born as early as next year. It sounds kind or creepy — or maybe a bit kinky — but the intriguingly polyamorous-sounding procedure is more DNA-swapping than wife-swapping.

Three-parent fertility treatment is a form of in vitro fertilization where a mother unable to have healthy children has the genetic information extracted from the nucleus of one of her eggs and combined with a donor's hollowed-out egg, which is fertilized by the father's sperm. Two mothers + one father = three parents.

The technique was developed so that women with mitochondrial diseases can have biological children. Only a tiny amount of the child's DNA would be from the third parent. Mitochondrial disorders passed down by affected mothers include fatal heart problems, muscular dystrophy, epilepsy, liver and kidney failure, brain disorders, and blindness.

Good intentions notwithstanding, the procedure is controversial. All procedures that involve messing with human reproduction are controversial, but throwing in a third biological parent muddies the ethical waters even further. CNN's Sanjay Gupta explains the controversy and the procedure:

Arthur Caplan, the New York University medical ethicist you're most likely to see quoted in any article on genetics, gives a qualified thumbs-up to the procedure. Thanks to Hollywood, people think "genetic engineering of animals or people is nothing but bad news," a sure path to eugenics and genetically created "perfect" babies, Caplan writes at NBC News. "But Hollywood is wrong," he adds:

The goal of the experiment in genetic engineering is not a perfect baby but a healthy baby. One in 4,000 children inherit terrible diseases due to genetic mutations in their mitochondria. The mitochondria are the batteries of cells. They provide the energy that lets cells divide and grow. All mitochondria are inherited from the mother — from her eggs. And, the mitochondria are separate, tiny units in the egg meaning you can pick them out and transplant them.... In my view, trying the technique to fix a terrible disease even with risks of failure makes ethical sense. [NBC News]

Caplan describes swapping an egg's mitochondria as giving the future baby "a new battery." The problem, he adds at CNN, is "what if you get past transplanting batteries and start to say, 'While we're at it, why don't we make you taller, stronger, faster, or smarter?'" The big question for the next 5 to 10 years is "how far do we go in pursuit of the perfect baby," he adds. "Do I think we're going down that road? Yes. Does it creep me out? Yes. Are you going to be able to draw a clear line? I don't think so."

And that's from somebody who thinks three-parent fertilization is worth trying out. There's always going to be a "slippery slope" question when we're meddling with human genetics — and where to draw the ethical boundaries is a question we should take seriously. In this case, there's also valid concerns about safety: Will swapping out an egg's batteries harm the resulting child? We won't really know until we try.

But these are the same kinds of questions we faced with in vitro fertilization 35 years ago, and more than 200,000 healthy babies have been born since, many of those to people who wouldn't otherwise have been able to be biological parents. There are still people who oppose IVF because they believe it's "playing God" to fertilize embryos in a petri dish, or — more commonly — because they believe human life starts at fertilization and IVF often creates more embryos than get implanted. For them, three-parent fertility is more of the same.

But there's something else going on. At the risk of setting up a straw man, here's the primary objection of Wesley J. Smith at National Review:

The purported reason for this approach is to enable women with mitochondrial disease — who would be the nuclear donors — to bear biologically related children. But you know it won't stop there. At some point, the process will be used to help, say, polyamorous threesomes or lesbian couples have children biologically related to all partners — that is to facilitate lifestyle desires, not prevent disease.... Isn't this the bane of our times! Just because a woman "wants" something, that doesn't mean we should turn ethics on its head so she can have it. [National Review]

Now, Smith brings up other points, too — the safety, the ethics of discarding fertilized eggs — but he keeps returning to that point: Three-parent fertilization is vaguely depraved and will quickly turn into a "lifestyle" procedure.

But that doesn't make sense. Who would go through a costly, invasive medical procedure to trade-up on mitochondrial DNA, even from — as Smith suggests — "beautiful and brilliant college women"? The only people this procedure is designed for are women who right now have the choice of not having biological children or taking a risk that their babies will have severe, even fatal, diseases. That's a pretty small number of women, but it's a pretty big deal for the families it affects.

In any case, we're likely to enter this brave newish world soon. On Thursday, Britain's Department of Health proposed new regulations that would allow three-parent fertilization in cases where there's a "significant risk" of passing on a mitochondrial disease. The proposals are open for public comment until May, and then Parliament can approve them (or not). Implementation will be up to the Human Fertilization and Embryology Authority.

The U.S. Food and Drug Administration is also weighing whether to allow mitochondrial transfers. An FDA advisory panel met Tuesday and Wednesday, without coming to any conclusions. But the United Kingdom will almost certainly get there first, for good or ill: The U.S. didn't approve IVF until 1981, three years after Britain's first "test tube baby" was born.