By Nicole Weisensee Egan
March 19, 2012 12:00 PM

Tyler Blackwell liked being an only child. But when the 18-year-old history buff learned he had 35 half siblings, conceived through the same sperm-donor father, “I was surprised and fascinated,” says Tyler, a college freshman in Frederick, Md. “I had read stories about this. I started picturing some giant family reunion.”

So far Tyler has met just one half brother. But his mother’s search for his biological father revealed that Tyler had inherited more than his donor’s blue eyes. He had a life-threatening heart defect that, if not corrected, could result in a fatal aneurysm. “I was terrified he was going to drop dead,” says mom Rebecca, 59. Such concerns were far from her mind when, single at 40, she selected her tall, blond, apparently healthy donor. “It was now or never,” she says. “I thought it was very safe.”

There are more than a million people alive today in the United States conceived by sperm donors, but because sperm banks keep donor identities secret and generally only screen for sexually transmitted diseases, most of these kids are missing key information about their medical histories. Now, though, moms like Rebecca Blackwell are at the forefront of a new movement calling for greater disclosure. “There’s not enough regulation,” says Sonia Suter, a law professor and bioethicist at George Washington University. “It’s the Wild West.” Adds Wendy Kramer, founder of the Donor Sibling Registry: “We don’t know how many kids are walking time bombs.”

Discovering those risks turned Blackwell into an unlikely medical crusader. In January 2010, after getting in touch with her donor, she learned he had the heart defect; her donor also had Asperger’s syndrome, something Tyler also has and Blackwell believes was passed on. Tyler had corrective heart surgery, but he’ll have to be monitored for life. Relieved about her own son, she grew terrified on behalf of the other 35 kids her donor had fathered: “I didn’t want children to die.” So she took it upon herself to make sure the three sperm banks where Tyler’s father-No. 832 to Blackwell-had donated were notified. (The donor declined to comment.) Sean Tipton, a spokesman for the American Society of Reproductive Medicine, which oversees sperm banks, says “human reproduction is a risky venture.” Blackwell’s efforts paid off, and then some. After spotting a news article about Blackwell, Rebeca Price of Santa Fe got in touch. Her son Stuart, 17, was Tyler’s half brother; he also has Asperger’s and a milder version of the heart defect. “I don’t think I will ever be able to express how grateful I am to Rebecca Blackwell,” says Price, 53. “She’s a lifesaver in every sense of the word.” Last month the two families met at Price’s home, and the moms were amazed. “They stand alike. They walk alike,” says Blackwell. Tyler, who is sending Stuart a chapter of a story he’s writing, doesn’t feel the need to meet his other half sibs, but he’s glad he’s found a brother. “We got along well,” he says. “It was pretty cool.”