DNA answer to Down's Screening

Scientists have found a simpler and safer way to screen a pregnant woman's baby for Down's and other similar genetic disorders. 

Currently the diagnosis of genetic abnormalities requires doctors to obtain samples of foetal tissue, either by drawing out amniotic fluid from around the baby and analysing the cells it contains, or by removing a small piece of placental tissue known as a chorionic villus sample (CVS).  In both cases there is a small by significant risk of miscarriage, and these forms of diagnosis also require a baby to be over about 14 weeks old (12 weeks in the case of CVS).

But now Steve Quake and his colleagues at Stanford in the US have developed a technique to do the same diagnosis on a sample of the mother's blood.  Their approach relies on the fact, discovered many years ago, that DNA (and to a lesser extent cells) from a developing foetus can "spill over" into the mother's bloodstream, where they can be analysed.

Writing in the journal PNAS the team analysed blood from 18 prospective mothers, 12 of whom were known to be carrying babies affected by Down's or two other conditions known as Edward's and Patau's syndromes.  Each of these conditions occur when the baby has an extra copy of one of its chromosomes.  The team diagnosed the condition by making copies of the DNA present in the mother's blood.  Using the human genome map they were able to work out which chromosomes the copies were from and tot them up.

In a normal pregnancy, where both the baby and the mother carry the correct number of chromosomes, there should be the same number of DNA copies from all the chromosomes; but if the baby carries an extra chromosome number 21, as occurs in Down's, then there will be too many copies of the DNA from chromosome 21, alterting doctors to the problem. 

Using this technique the researchers correctly identified all of the affected and normal pregnancies in the group of patients that they studied.  And apart from being less invasive and therefore safer for mother and baby, there is another benefit to this approach. 

According to Steve Quake the technique should work from as early as 5 weeks into a pregnancy, meaning that if termination is something a parent may elect to undergo they will know at a much earlier stage when the risks are lower and the process is much simpler.