The following is an excerpt.
A new non-invasive blood test for pregnant women could make it easier to catch abnormalities before their child is born.
Human cells should have two copies of each chromosome but sometimes the division is uneven. Existing tests count the fragments of placental DNA in the mother’s blood. If the fragments from one chromosome are unusually abundant, it might be because the fetus has an extra copy of that chromosome. But triploidy, where there are three copies of every chromosome, is missed, since the proportion of fragments from each chromosome is the same.
Read the full article here: New blood test finds elusive fetal gene problem