Somewhere between bites of cake and ice cream, Robin Anderson and her partner Cindy Brisco realized they had something major in common with another guest at a San Diego birthday party. Four years earlier, the pair had conceived a son with the help of the Fertility Center of California sperm bank. So had another guest at the party, single mother Maren Clausen. While comparing notes about their kids—Robin and Cindy's son Wade, then 3, and Maren's 3-month-old daughter Lila—conversation turned to respective sperm donors. "Ours was 48QAH," Cindy chimed in. "Yeah," said Maren evenly, as it dawned on her that—remarkably—she had chosen the very same donor. "The doctor who likes Sarah McLachlan."

The revelation that their kids shared the same biological father brought tears to Anderson's eyes. "I felt like I was in the middle of a miracle," she says. But there was another surprise to come. After registering on a Web site for donors and their clients, Anderson, 43, and Brisco, 44, soon learned the true identity of the man they had known only as anonymous donor 48QAH—Matthew Niedner, 34, a happily married critical-care pediatrician in Ann Arbor, Mich. Since then, Niedner has been in touch not only with them and Clausen, 43, but also yet another happy recipient of his donations—Mary Adsit, 45, a San Diego graphic designer who is now the mother of year-old Alexandra. And there could well be more little dividends in the future. According to the sperm bank's records—based on voluntary reports—Niedner's biological contributions have resulted in nine successful births—so far. "I do not consider myself to be these kids' father," he says, "but that doesn't mean I can't develop a relationship with them and their families."

Welcome to the brave new world of sperm donation. At the same time that families of the estimated 30,000 children born by U.S. donors each year have begun networking online—one Web site, the Donor Sibling Registry, claims to have put together more than 2,500 of them—some donors, like Niedner, are stepping out of the shadows to satisfy the understandable curiosity of families they have helped create. "For those who want to know the identity of the donor and who want a child to have a relationship with a donor, that ought to be available," says Sean Tipton of the American Society of Reproductive Medicine, an association of professional fertility specialists. "On the other hand, for patients who want to keep it private, that should be available as well."

In reality, the world of procreation by donor is neither so simple nor so problem-free. Though sperm donors are routinely tested for common genetic diseases like cystic fibrosis and sickle-cell anemia, just last month doctors reported that a sperm donor in Michigan (not Niedner) had inadvertently passed a rare and serious genetic disease to five children of four different couples, leaving the offspring vulnerable to infections and leukemia, and at a significant risk of passing the disease on to their own children.

There are other potential dark clouds on the horizon as well. What about the risk that biological offspring of the same donor will unwittingly marry each other? According to Niedner, who estimates he may have fathered as many as two dozen children in his six years as a donor, "nothing comes at zero risk, but statistically there's a low probability of that happening." And as for concerns about the psychological effect on progeny when they learn the true story of how they came to be, "it might not sit well with some of them," he concedes. "But no child gets to choose who their mother's partner is, what their creed is, or how much money they have. I think the vast majority would say they'd rather exist than not."

Niedner's reason for speaking openly about his role, he insists, is the same as the one that led him to become a donor in the first place: to help people. In late 1998, then a resident at the University of California, San Diego, medical school, he was talking to the mother of a young patient who had been conceived via donor sperm when she suddenly blurted out: "You should be a donor!"

"I laughed," recalls Niedner. "I said, 'Ha ha.'"

But the woman told him how difficult it had been to find a suitable candidate. And that, says Niedner with a straight face, "I guess is what planted the seed." Once or twice a month, he began stopping off at the Fertility Center of California, spending 20 minutes in a private room for $50 per session. "I thought, 'People donate blood and bone marrow,'" he says. "This isn't exactly the same, of course, but if I've got something of no particular use to me right now—that's of use to somebody else—it's a good thing."

He wasn't the only one who thought so. Based on the stats in his donor profile (see box), Niedner became a popular choice for prospective moms. Once, he asked what his donor ID stood for. "Finally, one told me," he says. He was the clinic's 48th approved donor, and QAH stood for Quite a Hunk. "I promptly turned bright red," he says.

By then, Niedner had met his future wife, Nicole, a third-year med student, but the two were not yet romantically involved. She supported what he was doing, and continued to, even after the couple married in 2002. There were other motivators as well. "It was kind of a feel-good thing, since the people at the clinic were always saying, 'Our clients are so happy,'" says Niedner.

Then, in 1999, Niedner saw an Oprah segment that mentioned the Donor Sibling Registry. "It talked about how these families have a lot of questions that aren't answered in the one- or two-page donor profiles," he says. "I got a sense of how psychologically important it was for these children to have their questions answered about their biological roots." Cautious at first, Niedner created an account that kept his anonymity but allowed contact by recipients. For six years he had no hits. Then, he says, last November an e-mail came from Robin Anderson, who heard about the site from a friend: "It said, 'Are you 48QAH? If you are, I have an incredible story to tell you, and I want to thank you.'"

Since then, Niedner and the four women whose children he helped conceive have done no more than trade photos and cordial e-mails. But soon there may be congratulations as well: after a miscarriage and three years of trying, Nicole, now a pediatric anesthesiologist, is pregnant with the couple's first child, due in early June. At the moment, Niedner, who last summer stopped his anonymous donations, isn't sure how much contact he'll have with the other children he has helped bring into the world—or whether he will ever even meet them face-to-face. "It will have to fit my life, and not threaten my family and my own children," he says. Just exactly what that means is something that he will have to play gradually, and by ear. Because, after all, as even a man of science like Niedner readily admits, "That's how human relationships develop."

I'm a happily married heterosexual without a lot of skeletons in the closet"