Todd Whitehurst was a young grad student at Stanford when he began donating sperm to pay his way through school.

"I went about 3.5 years, two or three times a week," said Whitehurst, who estimated he'd raked in more than $12,000.

Then six years ago, Whitehurst, 48, married with children and living in Los Gatos, Calif., was contacted by a 14-year-old girl who said she was his daughter and that she had obtained his information from the cryobank he'd visited.

Todd Whitehurst

"My first reaction was just to be stunned," Whitehurst said. "I would say I was kind of shocked at first but really happy about it overall."

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With more than 300 sperm banks expected to rake in a third of $1 billion this year, sperm donation births are a big business in the United States. ABC News learned of at least five individuals who had fathered more than 100 children and one individual who had fathered nearly 200.

"It's the wild, wild west of regulation," said Deborah Spar, president of Barnard College and author of "The Baby Business." "There's essentially no sheriff in town. There's virtually no regulation in this area, which has become quite large, quite lucrative and is literally involved in the most intimate area of people's lives."

Sperm banks test for about a half-dozen diseases, including HIV, but there are many more, such as diabetes, that they don't. There is no genetic testing and there is no limit on the number of children a donor can father, Spar said.

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In other countries, however, there are legal limits on the number of children a sperm donor can have - in the U.K., it's 10 - and children have a right to know who their biological fathers are and be able to obtain his health records.

"In England and most of Europe, it's illegal to have anonymous sperm donation," Spar said.

In the U.S. where there are more than 2 million children born from donated sperm, there are no official records that connect them to their biological fathers.

The private Donor Sibling Registry allows them to connect to their fathers and to their half-siblings, with their permission. In 13 years, the registry says more than 10,000 donors and children or siblings have connected to one another.

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Whitehurst was able to meet Virginia, his daughter, who found him via MySpace on her own. He also connected with three other children he'd fathered, thanks to the Donor Sibling Registry, and has confirmed a dozen donor offspring total.

He called the experience strange and surreal but also wonderful.

"The children I've met have been loved, have come from fantastic families and have just been some of the most fantastic kids I've ever met," he said. "Getting to know these kids has been the most wonderful opportunity, and I would definitely do it again if I had the choice."

For more on donor siblings, see Wendy Kramer's book "Finding Our Families."