FORT WORTH, Texas – The listing reads like a personal ad: “Brown straight hair. Dark brown eyes. Light brown complexion.”

But the writer isn’t looking for a date; he’s searching for his half-siblings and their common sperm donor.

Since 2000, when the Donor Sibling Registry was established, 18,604 people have posted their personal information. Some are children born through in vitro fertilization; others are parents, or donors. More than 4,700 matches have been facilitated.

Wendy Kramer of Colorado created the registry with her son Ryan, who was curious about his genetic origins yet knew little about his sperm donor.

When people go to the site, they often start out with mixed feelings about searching for a half-sibling or donor.

“They go into it thinking, “It would be great to find out I have a brother,”‘ she said. “Then they discover, “Oh my gosh, I have 22.”‘

The registry is full of donors. In the early days, men – many of them medical students – often donated their sperm.

Later, as the science of assisted reproduction developed, women began donating their eggs. The oldest donor on the site is in his 60s.

Unlike some countries that ban anonymous donations, the United States has always protected donor information. Those who want to search for a donor or siblings have only a sperm bank number to guide them.

For decades, children conceived with a sperm or egg donation were a well-kept secret. But over the years, as their numbers grew, people realized that this information needed to be shared.