ABC News’ Courtney Hutchison Reports:

A women  who conceives a child through a sperm donor  has to make her peace with a number of unknowns — what the donor looks like, what personality quirks he might have or whether big noses run in his family.  But one thing she probably didn’t bargain for was the possibility that her child could have more than 100 half-siblings out there, likely living in the same state, or even in the same city or neighborhood.

This was the outcome for Cynthia Dailey, who, through a little online research and networking, learned that her son had 149 half-siblings, all fathered by the same donor.  Her story, reported  by the New York Times this week, highlights a long-held concern among sperm bank critics: Shouldn’t we limit the number of offspring a single donor can sire?

As it stands now, there are no rules in place to monitor or limit how many times a single donor’s sperm can be sold — a situation that has allowed some sperm banks to oversell their donors, producing clans of more than 100 half-siblings.

ABC News covered this phenomenon last August, when we spoke to Chase Kimball, a sperm donor who, like the one Dailey used, likely fathered “hundreds of children” in the 1970s and 1980s.

It got the point where the clinic told him, “You’ve got too many kids locally, and we can only use your sperm if someone orders it from out of state.”

Having this many offspring is certainly not what Kimball and other sperm donors bargained for, and critics of sperm banks worry that allowing a single donor to father so many children will have negative ramifications for these children.

Because  most sperm donations are doled out to women living in the same general area, some critics argue that unintentional incest might become could occur.

Even more threatening may be what these “superdads” do to the gene pool, say some critics.  Sperm donors are tested for iniherited diseases to varying degrees – what they’re tested for differs from state to state —  but donors may still be passing along genetic abnormalities and diseases.

In July, ABC News covered the case of   Tyler Blackwell, an 18-year-old who, thanks to his mother’s curiosity and online investigation, learned that his donor father had passed on a rare genetic heart defect to him that could have killed him had it gone untreated.

In addition to the heart defect, the donor had the connective tissue disorder Marfan’s syndrome but failed to notify any of the three sperm banks where he had fathered at least 24 children, half of whom could be affected by these conditions.

The children of another donor with whom ABC News spoke were not as lucky.

Though this particular donor, who wished to remain anonymous, was screened for a number of conditions by the sperm bank where he donated, the tests failed to pick up the fact that he had hypertrophic cardiomyopathy, a genetic heart condition that can lead to heart failure and sudden death, among other problems. Of his 24 children (two from his wife, the rest from donation), nine have the genetic mutation responsible for HCM. One has functional limitations, chest pain and fatigue stemming from this mutation; another has heart palpitations; and one died of progressive heart failure at the age of 2, while awaiting a transplant.

“Sperm banks need to make an effort to collect updated medical information every couple of years,” Tyler’s mother, Rebecca Blackwell, told ABC News at the time.  “There is no one who knew about [the donor's heart condition],” she said. “If I could foretell the future, I would have picked a different donor. I didn’t know.”