Scientists have pieced together the entire DNA sequence of an 18-week-old fetus without having to use any invasive tests that could result in a miscarriage an advance that offers a glimpse of the future of prenatal testing.

Using blood drawn from the mother and a sample of saliva from the father, the researchers were able to scan the fetus' genome and determine whether it contained any of the myriad single-letter changes in the DNA code that can cause a genetic disorder. They could even pinpoint which mutations were inherited from Mom, which came from Dad, and which were brand-new.

If the technique is refined and the technology becomes inexpensive as many experts anticipate this type of prenatal testing could provide prospective parents with a simple, risk-free way to screen for a broad array of simple genetic disorders, according to the authors of a report in Thursday's edition of Science Translational Medicine.

The work is based on the fact that small fragments of fetal DNA circulate in the blood of pregnant women.

Several biotech companies are developing tests that capture those DNA fragments and screen them for signs of Down syndrome and other disorders that result from having an extra copy of an entire chromosome.

But that type of screening is far easier than searching for single-letter variations in individual genes, said senior author Jay Shendure, a geneticist at the University of Washington in Seattle.

An additional chromosome is "the equivalent of an extra chapter in a book," he said. "What we're trying to do is pick up a typo in a word."

To set about their task, Shendure's team started by sequencing the genome of an anonymous pregnant woman, using a complete sample of her DNA obtained from her blood cells. They also sequenced free-floating DNA fragments extracted from her blood plasma, repeating their work until they had decoded every part of the human genome 80 times.

That plasma contained a mix of 10% fetal DNA and 90% maternal DNA, all in tiny fragments. The scientists needed to be able to tell which pieces were from the mother and which belonged to the fetus.

To solve that problem, the scientists relied on the fact that genetic material is inherited in long strands of DNA, called chromosomes and that tiny genetic variations on the same chromosome are usually inherited together, in blocks known as haplotypes. If a given haplotype was present in the fetus as well as in the mother, it would be detected in the plasma in extra amounts.

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Entire DNA of fetus revealed through risk-free testing