Parents of test tube babies try and seek out their children's siblings.
Parents who conceived with donated sperm or eggs are increasingly seeking other families who used the same genetic material, sometimes locating as many as 55 "siblings" for their offspring, a study found on Tuesday. The findings published in the journal Human Reproduction raise the issue of reusing a single donor's sample numerous times -- something policy makers may soon need to address, the researchers said.
In some cases, parents found more than 10 donor siblings, and one parent found 55 brothers and sisters for their child, Tabitha Freeman of the Center for Family Research at the University of Cambridge in Britain, who led the study, said.
More than 90 percent of parents included in the study came from the United States, where guidelines regulating the use of sperm or eggs are looser than in Britain, she added.
"The study is exposing that some clinics are using the same donor for a lot of families," Freeman said in a telephone interview.
"Guidelines suggest this should not be the case but they are not strictly enforced" in the United States, she added.
The findings have implications for policy governing a donor's right to anonymity, and could spur legislation limiting how many times genetic material from one donor can be used as a fertility aid, the researchers said.
This is especially true as an increasing number of women are waiting longer to start having children, creating more demand for in vitro fertilization to help them conceive.
"Our most important finding is that the practice of donor conception is creating new family forms," Freeman said. "These family forms are based on genetic links between families with children conceived by the same donor."
Freeman and colleagues recruited 791 parents through the Donor Sibling Registry, a U.S-based group which allows families who used the same genetic material to find each other.
The parents, who had signed up to a service which matches donor samples to parents, were able to discover the identity of their children's "donor siblings" -- other children who shared their genes but were born to different families.
"One very striking finding is that family members in this sample formed close links based on notions of family and kinship," Dr Freeman said. "For example the mothers experienced maternal feelings toward their children's donor siblings."
Since 2005 in the United Kingdom, donors have had to agree that they are willing to be identified by parents who used the sample or their children when they turn 18 -- a reversal of previous legislation which guaranteed anonymity.
The study authors said further research was required to assess the long-term psychological impact for donor offspring of contacting and meeting many of their donor siblings.