When you get pregnant from your twin's ovary, who's the mom?

When you get pregnant from your twin's ovary, who's the mom?

When you get pregnant from your twin's ovary, who's the mom?

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Nov. 20 2008 6:39 PM

Children of the Clones

When you get pregnant from your twin's ovary, who's the mom?

What's the next best thing to having your own baby? Having your identical twin's baby.

A woman in England just did it. Her ovaries didn't work, but her sister's did. So doctors transplanted an ovary from the fertile sister to the infertile one. The result, announced a few days ago, is the first baby verifiably born from a whole-ovary transplant. The story raises a bunch of messy questions, starting with this one: Who's the mom?

William Saletan William Saletan

Will Saletan writes about politics, science, technology, and other stuff for Slate. He’s the author of Bearing Right.

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If you get pregnant with a donor egg, you're the gestational but not the genetic mother. But what if the donor is your twin? It's easier to think about this in the context of organ transplants we're already familiar with. Suppose you get a kidney transplant from your identical twin sister. Genetically, your new kidney is (almost) the same as your old one. The new kidney wasn't born in you, but you and it developed from the same embryo. Not just the same womb, but same embryo. In that sense, it really is yours.

Eggs and ovaries are more complicated. Your twin sister's ovary, like her kidney, came from the same embryo that produced you. Because of reproductive cell division, any one of her eggs would differ genetically from any one of yours (though even that point is quite complicated). But over time, her ovary and yours will yield almost the same set of eggs, if not in the same order. It's as though each of you rolled the same pair of dice a million times. So when she gives you an ovary instead of an egg, the result will be as though you were getting back your original ovary.

And that's the point. Doctors are choosing twins for these pioneering ovary transplants not because it's cool or weird, but because what's cool and weird about your twin—that she's genetically identical to you and yet is a different person—is also medically crucial. One reason it's crucial is organ rejection. Ovaries, unlike kidneys, aren't necessary for survival. If you got an ovary transplant from a random woman, you'd need serious drugs to stop your body from rejecting it as foreign. The rejection or the drugs could harm or even kill you. But if the ovary comes from your identical twin, it's not foreign. Your body accepts it. This is much safer.

The second reason is that your twin, while genetically identical to you, is physically distinct. This is important because the primary purpose of twin ovary transplants isn't to help twins (there aren't that many) or to advance toward ovary transplants between strangers. The primary purpose is to perfect the best kind of transplant: the kind you get from yourself.

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At first glance, this sounds nuts. If your ovary works, why take it out in the first place? There are two answers. One is that tens of thousands of still-fertile women have to get chemotherapy or radiation for cancer. They'd rather not nuke their eggs as part of the deal. By removing an ovary, freezing it during cancer treatment, and later restoring it, they stand a decent chance of beating the cancer and still having kids.

The other answer is that millions of women would like the freedom to delay motherhood beyond the years nature intended. That's the ultimate market for ovary transplants, according to Sherman Silber, the doctor who did the procedure on the woman from England *. "Women have opportunities they didn't have before, they do not want to commit to a relationship until they are sure it is the right one, they want to get the degree, save a little money and buy the nice flat," he told the Telegraph. Today, these women risk losing their fertility. IVF is expensive and uncertain. Donor eggs are hard to get, and the child isn't genetically yours. A self-ovary transplant, Silber points out, is "so much nicer and more convenient."

In fact, self-transplants of partial ovarian tissue have already been done. The problem is that when the woman subsequently gives birth, doctors can't be sure whether the egg came from the reimplanted tissue or from tissue left behind. The only way to be sure is to take out the whole ovary or, better yet, to get the ovary from somebody else. Somebody else who matches you genetically. Your twin. That's why doctors working on self-ovary transplantation are so excited about the progress in twins. It's a testing ground, says one, to see whether "the entire organ can be successfully retransplanted."

Down the road, this research raises big questions. What's going to happen to us as we detach motherhood from what were known, back in the 20th century, as the fertile years? What happens when we can put those years in a freezer and shoot them into the future? It's great that women can have careers, take their time finding the right person, and just be themselves before starting a family. But if we think we've stopped the biological clock, we're kidding ourselves. It's more like that time-travel scenario where you send the astronaut into space at nearly the speed of light and he comes back a year later to find that everyone else has aged a decade. The clock in your frozen ovary slows, but the clock in your body keeps on ticking. The ovary comes back to an older, weaker host. Maybe you can still have a baby. But can you raise it?

For now, we're still working on twins. By last year, Silber had done ovarian tissue transplants between seven pairs of twins. More whole-ovary twin transplants will follow this one. Three years ago, the former procedure produced its first baby; now, the latter procedure has done the same. Who exactly are the biological parents of these children? "I haven't really spent any time thinking about the idea that I am the genetic mother," says the twin who donated the ovary for the child just born in England. Is she the genetic mother? Or is it her sister, who carried the child and came from the donor's embryo? Or is it both?

We've heard of scenarios like this before, but in a different context. Seven years ago, in his maiden speech on stem-cell research, President Bush warned that human cloners might "grow another you, to be available in case you need another heart or lung or liver." Critics called this science fiction. But such clones already exist. They're called identical twins. They've given each other kidneys, liver tissue, even hearts. Now they're giving each other babies. In this miracle of love and science, an embryo that split in the previous generation reunites in the next. "I always say she is the other half of me," the donor in the English case says of her sister. And so she is.

Correction, Nov. 24: The article originally said the ovary transplant was performed in England. Actually, although the patient lived in and gave birth in England, the transplant was performed in the United States. ( Return to the corrected sentence.)