One Sperm Donor, 150 Offspring

“It’s wild when we see them all together — they all look alike,” said Ms. Daily, 48, a social worker in the Washington area who sometimes vacations with other families in her son’s group.

As more women choose to have babies on their own, and the number of children born through artificial insemination increases, outsize groups of donor siblings are starting to appear. While Ms. Daily’s group is among the largest, many others comprising 50 or more half siblings are cropping up on Web sites and in chat groups, where sperm donors are tagged with unique identifying numbers.

Now, there is growing concern among parents, donors and medical experts about potential negative consequences of having so many children fathered by the same donors, including the possibility that genes for rare diseases could be spread more widely through the population. Some experts are even calling attention to the increased odds of accidental incest between half sisters and half brothers, who often live close to one another.

“My daughter knows her donor’s number for this very reason,” said the mother of a teenager conceived via sperm donation in California who asked that her name be withheld to protect her daughter’s privacy. “She’s been in school with numerous kids who were born through donors. She’s had crushes on boys who are donor children. It’s become part of sex education” for her.

Critics say that fertility clinics and sperm banks are earning huge profits by allowing too many children to be conceived with sperm from popular donors, and that families should be given more information on the health of donors and the children conceived with their sperm. They are also calling for legal limits on the number of children conceived using the same donor’s sperm and a re-examination of the anonymity that cloaks many donors.