Even with genetic evidence that a child's mother and father are biologically related, "you can't tell from the test definitively who the biological father is," McGuire said. "If there's multiple first-degree relatives, you wouldn't be able to discern whether it was dad, or a brother. That's what makes it difficult to develop general, definitive practice guidelines. Like most areas of medicine, it really depends on the circumstances of the case before you."
In 40 years of doing clinical genetics, Beaudet said he occasionally saw a child whose mother and father were close relatives. But growing use of genetic testing that analyzes single-nucleotide polymorphisms (SNPs, pronounced "snips"), which are variations in single building blocks of DNA, can inadvertently reveal the secrets of a child's birth. That is forcing health professionals to consider the most ethical ways to handle potentially incriminating or otherwise damaging information.
Beaudet said that because doctors are legally compelled to report child abuse to authorities, they need to report incest involving underage mothers, and must ascertain if older mothers were minors at the time of conception. Hospital committees that handle suspected abuse must consider how to protect a mother if she risks returning to an abusive home life.
"Mostly we're dealing with young girls in the home who are probably being sexually abused by their father," Beaudet said in an interview. "They're carrying the babies to term, and they're probably being told that if they tell anyone about anything, they'll be harmed or injured.
"It's not so unusual to have a girl have two or three children with her father," Beaudet said. "If you discover the first one, there's some potential to stop the second one."
He cited one case in which "there were at least three and maybe four girls who were having multiple children with their father. He was in and out of jail. The children were showing up at genetics clinics for obvious reasons."
Beaudet and his colleagues decided to go public to discourage, if not stop incest. "Maybe the people who are the offenders are not amenable to the kind of education we'd like to get out, but ... if this happens and there's a child born, it may well uncover this and get the offender in trouble."
Lynn Jorde, president of the American Society of Human Genetics, said a society committee that deals with social, legal and ethical issues will consider the legal and ethical ramifications of "detectable incest, especially when there is the possibility that someone may have been abused as a minor." He said it could be "scientifically important to know...if we take a large collection of children with, say, developmental delay, how often do we see incest or just consanguinity?"
Jorde, chairman of human genetics at the University of Utah School of Medicine, said that in most cases where first cousins mate, "most offspring are perfectly healthy," and he worried that some of the genetic test results could make families worry unnecessarily.