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When people trying to have a child use a sperm or egg donor, they usually sign an agreement. It says they promise never to seek out the donor's name. But as NPR's Joseph Shapiro reports, a growing number of parents and children want to know more about their extended hidden families.
JOSEPH SHAPIRO: Donor siblings - that's the term for people who are conceived with the egg or sperm of a common donor. Every donor is given a number and that number is public information. There's a place on the Internet where people can share these numbers and voluntarily seek out donor siblings. It's called The Donor Sibling Registry and it was started a little over eight years ago by Wendy Kramer.
Ms. WENDY KRAMER (Donor Sibling Registry): And I posted a very short message that said I'm the mother of an awesome 10-year-old donor child. I know that he has at least three donor siblings and would love to contact them. We are looking for donor 1058 from California Cryobank.
SHAPIRO: Today, Kramer's son, Ryan, knows of six half sisters, all conceived with the sperm of donor 1058. Two years ago, Wendy Kramer and her son flew from Colorado to New York to meet one of them, a girl named Anna.
Ms. KRAMER: And the meeting was very emotional. It's kind of weird, I mean, I know Anna is nothing to me. I'm not related to her. But when I saw this little girl who had parts of my son in her, there was an immediate connection with her and with her mom. I mean, we hugged each other and we cried.
SHAPIRO: A few weeks later, a Mother's Day card came in the mail from Anna.
Ms. KRAMER: And on the front of it, it was M.O.M. and it said, My Other Mother. And I think Anna as a 13-year-old at the time, she was also trying to define -well, wait a minute, what are we two each other? So, Ryan is my brother, but what are you and how - you know, we were all just kind of trying to figure out what does this all mean and how do we go forward?
SHAPIRO: They're not alone. More than 6,000 people have been linked through the Donor Sibling Registry. It's a self-selected group of people who do want to find each other. Most families that use assisted reproduction don't seek out donor siblings. The ones who do are usually single mothers like Wendy Kramer or lesbian couples. Tabitha Freeman is a researcher at the University of Cambridge in England, and she's been studying families on the Donor Registry.
Ms. TABITHA FREEMAN (University of Cambridge, England Researcher): It's a new form of family. It's not, perhaps, what people traditionally understand by families, sort of mom, dad, child. And yes, they do describe each other as brothers and sisters.
SHAPIRO: It's often assumed that people seek out donor information to learn about their own medical history. But that's not what Freeman found in the study published in the current issue of the journal, Human Reproduction. Instead, people said mainly they were curious - that was Wendy Kramer's reason. Or they wanted to help their child get a sense of identity.
In Freeman's survey, the average number of related donor siblings who found each other was five. But some of those on the registry have 20, 50 or even in one case, 120 donor siblings.
Ms. FREEMAN: I think when you get to a certain number it's just a very different understanding of what a sibling is. So, we do need research on this sort of bonds between these larger numbers.
SHAPIRO: One day last summer, Wendy Kramer logged onto her computer.
Ms. KRAMER: Ryan and I were sitting in the living room. I had my laptop. He was sitting on the other couch. I signed on and I went, Ryan, and he knew that tone of voice, and he just looked at me and he said, boy or a girl? How old?
SHAPIRO: This summer Wendy and Ryan Kramer plan to fly east again to meet those two new half sisters.
Joseph Shapiro, NPR News.