New blood test finds elusive fetal gene problem

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.

California-based company Natera uses an algorithm to calculate the most likely genotype for the fetus. To do this it looks at single letter variations called SNPs in the parents and compares this to a database of the most common SNPs patterns in the population. This genotype is then compared with placental DNA.

This approach can catch triploidy since the whole fetal genotype is the reference rather than a single chromosome. The method was presented last week at the Society for Maternal-Fetal Medicine in San Francisco.

This article appeared in print under the headline "No hiding place for fetal gene errors"

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