Your Baby, Bit By Bit

By Medical Discovery News

Oct. 6, 2012

Your Baby, Bit by Bit

Now, women wait longer to have their first child than ever before. National health statistics report the number of women having children after age 40 has quadrupled since 1980. As a result, when they are ready, women often worry their baby will carry a genetic defect. Today a variety of genetic tests are available, mostly by screening the mother’s blood for specific fetal proteins and hormones. But many of the test results can’t be confirmed until a doctor performs Chorionic villus sampling (CVS) or an amniocentesis. Both are invasive and carry a relatively high (1 percent) risk of miscarriage.

For over a decade scientists predicted a noninvasive, risk-free technique was close because fragments of fetal DNA are present in the mother’s blood. Henry Lo of the Chinese University of Hong Kong, who made the initial discovery, also wrote in a paper two years ago that there’s enough fetal DNA fragments to not just screen for genetic diseases, but to construct the fetus’s entire genome.

Recently researchers from the University of Washington proved this theory by developing a method to assemble these DNA fragments and successfully sequenced the genome of an 18.5- week-old fetus. Later, when the baby was born and traditional sequencing was done, a comparison of the two showed the researchers had been 98 percent accurate, a major breakthrough in fetal genetic testing.

A major challenge for researchers was differentiating fetal DNA fragments from the mother’s DNA, which took three steps. The first was obtaining the father’s saliva to yield his genome sequence. From the mother’s blood, the maternal genome was decoded, down to reading the DNA sequence in each of her 23 pairs of chromosomes.

Then researchers isolated all the DNA fragments floating in the plasma portion of the mother’s blood, of which 10 percent belongs to the fetus. Having sequenced the genomes of both parents, researchers could then pick out DNA that varied from the parental DNA sequence and reassemble them to form the fetal genome.

The method worked on a fetus in the second trimester, and researchers believe as they fine tune the technique it may be possible to start as early as six weeks after conception.

A major limitation now is the cost of fetal genomic sequencing – $50,000 per child. But scientists predict, with the price of genomic sequencing continuing to drop, the test will eventually become available in doctor’s offices.

Though parents could choose to have the entire fetal genome sequenced, it still won’t predict or rule out all genetic diseases because scientists have identified just a fraction of all the genes responsible for birth defects. To complicate matters, some abnormalities are not gene related, and certain genetic mutations only predispose a baby to disease.

Geneticists will continue to identify disease-causing genes, but eventually they will also discover genes behind traits such as intelligence and athletic ability. The use of that information will become an ethical dilemma.

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