Half-siblings conceived with donated sperm and eggs are connecting online using DNA testing and online registries, forming extraordinarily large genetic families with dozens to hundreds of children linked to one parent, The Washington Post reports.

The modern family ties and genetic sleuthing are making it easier for donor-conceived children to learn about their backgrounds—and harder for anonymous donors to maintain anonymity . That has clearly been proven in tragic cases in which fertility doctors misled patients about their donor’s identity, even using their own sperm to sire dozens of children . But in legal, less-scandalous cases, the online connections are also highlighting the complex consequences of America’s lax regulations of the fertility industry, particularly on sperm and egg donations.

Many other countries have set legal limits on the number of children, families, or pregnancies to which one donor can contribute. Sperm donors in Taiwan can only sire one child, for instance. In Britain, they can donate to 10 families, and in China they can provide starter material for five pregnancies. But in the US, no such limits exist.

The nonprofit organization the American Society of Reproductive Medicine recommends limiting each sperm donor’s contributions to 25 births within a population of 800,000, which is about the size of San Francisco. As the Post points out, that could allow for one donor to sire more than 10,000 children across the entire country.

Though that number may seem absurdly large, the real numbers are also eye-popping. In one instance, half-siblings used online registries and DNA testing to discover that their biological father, sperm donor #2757, sired at least 29 girls and 16 boys. The half-siblings range in ages from 1 to 21 and live in eight states and four countries. Other sibling networks linked online ranged in size from dozens to nearly 200.

Such large genetic families raise concerns about half-siblings meeting unknowingly, falling in love, and having children of their own, risking genetic disorders. The vast family connections also exacerbate concerns that donors are often not required to provide medical histories and updates. There are already cases in the medical literature of half-siblings discovering each other while seeking treatments for rare genetic conditions, the Post points out.

Last month, the US Food and Drug Administration rejected a petition put forth by donor-offspring that sought to limit the number of births to which a donor could contribute. The petition also urged the FDA to track the number of each donor’s offspring and make collecting donor medical histories and updates mandatory. The FDA responded by saying that such oversight extended beyond its authority, which for now is limited to making sure that donors are screened for certain infectious diseases.

The response has infuriated families with donor-conceived children who want more regulations and transparency for donors. Meanwhile, donor-offspring continue to link up online. One daughter of donor #2757 told the Post:

“Every time I find a new sibling, I get anxiety and think to myself: when is it going to end?”