When Katie Trebing's parents considered creating a sibling to try to cure their daughter, they had few other families to seek out for advice -- the two geneticists who perform most of the tissue matching in the United States estimate only between 100 and 200 such donor children exist worldwide.

The first time preimplantation genetic diagnosis (PGD) successfully yielded a donor child was in 2000. Because the scientists who perform PGD are fertility and genetic specialists, a different team of experts performs the subsequent bone marrow transplant. So nobody knows how many siblings actually provided a transplant or whether they were successful.

Transplants can be done using either the new child's umbilical cord blood or bone marrow extracted from the donor child's body. No person or organization in the United States is known to be collecting data on which choice doctors are making.

Most of the information on what are called "savior" or "donor" siblings in the United States is anecdotal, gleaned from doctors or geneticists who follow up with families and who say they have not seen any abnormalities. Studies of children who had PGD as embryos are being conducted in Belgium and in England.

Sibling matching accounts for only about 1 percent of all PGD cycles, according to data voluntarily collected from U.S. fertility doctors for the first time in 2006 by the Genetics and Public Policy Center at Johns Hopkins University in Maryland. The rest of the time PGD is used to test embryos for inherited diseases or for chromosomal abnormalities.

A PGD cycle is counted each time a woman uses PGD to try to get pregnant during one menstrual cycle. If she fails to get pregnant and has to try again the following month, that is counted as a separate PGD cycle.

Of 3,000 PGD cycles reported in 2005, 43 were for donor matching, said Susannah Baruch, director of reproductive genetics at the center. The questionnaire didn't ask whether any babies were actually born after those cycles, Baruch said.

Conceived to save her sister

The concept of creating a donor sibling didn't start with PGD -- in 1990, the Ayala family in Southern California publicly declared that they were conceiving a child in the hopes it would be a match for their then-16-year-old daughter, Anissa, who had leukemia and needed a bone marrow transplant.

But the Ayalas were taking a chance -- hoping the new baby would be a match for their daughter. And she was. At 14 months of age Marissa Ayala donated marrow to her sister, Anissa.

"What they did was really, completely just a blessing," Anissa Ayala, now 35, said of her parents. She works for the National Bone Marrow Registry in Southern California. "My sister is the one who will tell you that without me she wouldn't be here, and without her, I wouldn't be here." Marissa Ayala did not want to be interviewed for this story, Anissa Ayala said.

"She doesn't really want to focus on that as far as who she is," Anissa Ayala said.

With the evolution of PGD, parents wanting an exact match sibling could virtually guarantee it before a pregnancy began. As the PGD process becomes less expensive and less invasive, many more children with bone marrow diseases such as leukemia could benefit from donor siblings, reproductive experts said.

PGD used to test for diseases

Historically, PGD was developed to determine whether embryos had inherited certain diseases their parents carried, such as sickle cell anemia, according to Mark Hughes of Genesis Genetics Institute in Detroit, one of the nation's foremost experts on the testing.

Before 1990, doctors told parents of children with inherited diseases that any future child would have a 25 percent chance of having the same disease, and that they would have to roll "the genetic dice" if they wanted another child, Hughes said.

If they went ahead, the couple would go through 15 weeks of anxiety before a test of the amniotic fluid, called amniocentesis, could show whether a fetus had the disease, Hughes said. If yes, the parents would decide whether to abort the fetus.

Hughes and other doctors wanted to push the diagnosis back to before a pregnancy began. The result was PGD, which allows for a test done on an embryo before it's implanted into the mother's womb.

An estimated 10,000 children worldwide are thought to have been born using PGD, but no American organization tracks these births. The oldest of these children is now 17.

PGD also used to select donor

What evolved from routine testing was using PGD for the benefit of a sick sibling who needs a bone marrow transplant. Hughes said he has performed PGD for tissue matching that has resulted in 114 births. Yury Verlinsky, director of Chicago's Reproductive Genetics Institute, said his work has resulted in the births of 40 donor siblings and estimates an additional 40 have been born in Europe.

A committee of American scientists has been meeting through academic and professional organizations for the past two years to try to implement a U.S. registry to track PGD in all its uses, said David Adamson, director of Fertility Physicians of Northern California and president-elect of the American Society for Reproductive Medicine.

The Genetics and Public Policy Center at Johns Hopkins University is working with the American Society for Reproductive Medicine and the Society for Assisted Reproductive Technology to set up a registry in the next year or two. Adamson is on that PGD committee, which has been meeting about three times a year for the past two years.

Scientists involved with PGD for sibling matching say they'd like to know the success rate of producing donor children, how many subsequent transplants are done with cord blood vs. bone marrow, how many times in the future a donor child is called upon to aid the sibling, and future health or psychological problems for the PGD child.

"I can absolutely guarantee that the scientists and the clinicians who are involved with this want this," Adamson said.

Database of donors urged

A registry of PGD offspring is the least that should be done, and it should be sure to isolate children conceived as donors into their own category, said George Annas, a professor of health law, bioethics and human rights at Boston University.

"So we can say in 10 years, 'Is this good for kids, or is this bad for kids?' I'm not saying I'm against it, but I'm in favor of keeping statistics on it and protecting the child -- the donor -- from exploitation."

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