The scientists also couldn't predict what the child's genome looks like at roughly 1 million points where both parents had differences between their chromosome pairs. That's quite important for prenatal diagnosis in marriages between "blood relatives" such as first or second cousins, common in many cultures, Lo says. The two parents could each be carriers of a disease-causing mutation at the same spot due to their shared inheritance, he notes, and any offspring inheriting both mutations could then develop the condition.
Nonetheless, other geneticists are impressed. Arthur Beaudet, of Baylor College of Medicine in Houston, calls the new study "outstanding work" and predicts that "the near future will involve the routine sequencing of the genomes of fetuses during the first trimester of pregnancy."
Improving the technique to make it a clinical reality will only take "a couple of years," says Shendure, who estimates that his study cost about $50,000 per child. But he says it would be naive to think that all the data could actually be useful medically at that point. "There will be many mutations whose impact we just don't know."
Hilger Ropers, a geneticist at the Max Planck Institute for Molecular Genetics in Berlin, agrees. The genetic causes of most developmental disorders haven't been found yet, he cautions. In the meantime, he advises scientists to put more effort into finding the causes of such conditions. "After all, we can only eliminate those genetic disorders that we know."
Even though it should be possible to sequence a fetus's whole genome, Lo says, it might be better to do it in a targeted fashion so that information doesn't overwhelm prospective parents. That might also avoid some difficult decisions, such as whether to abort a baby who has mutations that forecast a difficult future. For example, Lo says: "I don't think it would be ethical to use this to screen for late-onset diseases like Alzheimer's or cardiovascular diseases, for example."