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Children of the ClonesWhen you get pregnant from your twin's ovary, who's the mom?

By William SaletanPosted Thursday, Nov. 20, 2008, at 6:39 PM ET

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

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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?

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.

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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.