When Maggie was 12, she was excited to go to sleep away camp for the first time. Though she’d never really been away from home before, she had no idea that the experience would completely change her life — and introduce her to a half-sister she never knew she had.

Maggie, from San Francisco, met a girl, Cathy, at camp who said that Maggie reminded her so much of her best friend back home; they looked alike, and had the same mannerisms. When Cathy discovered that Maggie had been conceived with the help of a sperm donor, she wondered if the two could even be related, as her friend back home had also been born through a sperm donor.

Turns out, they were sisters. And their story is not as rare as you’d think. There are countless tales of donor-conceived siblings meeting each other accidentally. Random meetings have happened at parties, school functions, parks, even on a Disney Cruise.

The large numbers of offspring conceived with the help of sperm donors is sparking fear that half-siblings could mistakenly have sexual relations with each other

In another case, two women who met while searching for a roommate at Tulane University in New Orleans discovered that they had similar backgrounds: Both had lesbian moms who had used a sperm donor. The students both had long, curly brown hair and chin dimples. After joking for a semester about being “sisters,” they finally asked their mothers for their sperm donor identifying numbers — and discovered that they actually were half-sisters, too.

Now, the large numbers of offspring conceived with the help of sperm donors is sparking fear that half-siblings could mistakenly have sexual relations with each other, without knowing their true connection. Some kids, such as Maggie, are now taught to memorize their donor numbers in case they want to date someone who was also born via a sperm donor.

One reason this is such a legitimate risk: There are no limits on the number of children born to each sperm donor.

In fact, one American donor is known to have fathered at least 150 children, and there are many other donors with large groups of offspring. Some women who used sperm banks have been shocked to find out how many children their donors produced.

For the kids, trying to connect with dozens of half-siblings can be daunting. There’s also concern that finding out that he has 50 or 100 kids might make a sperm donor less likely to reach out to them.

Since no one is keeping track of the children produced, it’s difficult to alert families when it’s discovered that a donor has a rare, genetic disease.

Wendy Kramer created a website, DonorSiblingRegistry.com, to help her son find his half-siblings and has seen these groups grow in size.

“There are many half-sibling groups on the Donor Sibling Registry that are between 100 to 200 [kids],” she says. “Because the DSR is a voluntary registry, we can assume that these groups are actually much larger in size, as not everyone is interested in connecting. Additionally, when these groups get too big, we see people removing their posts, so it’s hard to know exactly how large many of the groups actually are.”

Kramer and other experts believe that establishing donor limits would help. In the U.K. and Sweden, for examples, once a child is born to a tenth family via the same sperm donor, the donors sperm is removed from banks and he is not allowed to donate any more. As long as there is accurate reporting of births, this is entirely possible. At the moment, there is no U.S. laws regarding this — only guidelines established through the American Society for Reproductive Medicine, a professional organization, that suggests a limit of 25 children per population of 850,000.

Obviously, that’s not working.

The fact that there is also little tracking of births by sperm banks and clinics — mothers are asked to self-report births to the clinics, but not all do so — is another concern. Since no one is keeping track of the children produced, it’s difficult to alert families when it’s discovered that a donor has a rare, genetic disease. While sperm banks do test donors for a variety of genetic illnesses, they can’t check for all of them, and there are multiple cases of children discovering that they have serious diseases inherited from their donors.

It’s difficult to alert families when it’s discovered that a donor has a rare, genetic disease.

In one particularly tragic case, a baby in Oklahoma was found to have inherited cystic fibrosis from his donor, who was a carrier for the condition. The man was apparently never checked for the disease, possibly because the sperm was 20 years old. (The parents were also not told how old the sperm was, which is problematic.) A progressive disease, cystic fibrosis causes persistent lung infections and limits the ability to breathe over time. The median survival age is around 40. Now the boy’s parents fear they will outlive their own child.

Despite all these issues, the fertility industry in this country is almost entirely unregulated. It may be difficult to convince sperm banks to create limits on their own — especially since they are making so much money. The global sperm bank market is expected to reach $4.96 billion by 2025, according to a new report by Grand View Research, Inc, a market research firm.

That’s why we need legislation now to address these issues, including limits on the number of children a donor may have, stricter medical and genetic testing of all sperm donors, and a national registry for donors and their biological offspring that would keep track of them all. Children born through sperm donation are vulnerable and need to be protected.