Safer Prenatal Testing

Testing for Down syndrome may get much safer in the near futureA new study out from Stanford (yay Stanford!) promises to revolutionize prenatal testing. Instead of looking at a fetus’ cells, the test looks at mom’s blood. More specifically, it looks at the small amounts of fetal DNA found in her blood.

This test will get around the small but very real risk of miscarriage that comes with more invasive forms of testing like amniocentesis. Instead of inserting a needle through mom’s belly and into the amniotic sac, a doctor will insert a needle into her arm. This is obviously much safer for the fetus.

The test won’t be able to find everything though. It can’t find subtle genetic mutations like those that cause cystic fibrosis or sickle cell anemia.

What the test can do is identify fetuses with extra chromosomes. This is one of the main reasons women get prenatal testing--to screen for diseases like Down, Edward's or Patau syndrome. Each one of these is due to an extra chromosome that the test can detect.

The test is actually marvelously simple. A sample of blood is taken from mom and the cells are removed. The researchers then take a close look at the DNA that is left behind. Most of this DNA comes from mom but some comes from the fetus too.

The researchers then figure out what millions of bits of the DNA look like. Next they match the bits of DNA to the chromosomes they came from. If more of the DNA bits match a certain chromosome than expected, then there is most likely an extra copy of that particular chromosome.

Think about it this way. Imagine a bag full of 23 different colored marbles where each color represents one human chromosome. You pull out marbles millions of times and then tally up the colors you get. (This can either be a ginormous bag or you can put the marbles back in once you pick one.)

If you pick more blue marbles, then there were probably more blue ones in the bag to start with. Of course this only works if you pick marbles many, many times.

Instead of using marbles, the researchers looked at the millions of bits of DNA they collected. They found that if there was an extra chromosome, then DNA bits from that chromosome were overrepresented. For example, they found that the 9 Down syndrome pregnancies they looked at had anywhere from 4-18% more chromosome 21 DNA than normal pregnancies*.

The 4% number was good enough to find pregnancies with extra chromosomes in this small study. They’ll need to repeat it in a larger study to turn this great idea into a diagnostic test for the public.

The test costs about 700 dollars to run right now but as sequencing gets cheaper (and it will), that cost should go way down. Also, right now they had to wait for the 14th week (which is a bit earlier than the 15-18 weeks for an amnio). Hopefully they’ll be able to improve the method to get testing done even earlier.

*The reason they didn’t get 50% more (as you might expect from having an extra chromosome) is that the fetal DNA made up only a small fraction of the DNA in the sample. In other words, most of the DNA was mom's.

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