[bobdihi (OP)]

How far into pregnancy can you test the fetus DNA?

[chris]

Historically this procedure was performed from about 15 weeks' gestation by collecting fluid from around the developing baby. This is called "amniocentesis" and involves passing a long needle through the abdominal wall and into the amniotic sac within which the baby develops. Foetal cells, shed naturally as the baby forms, are present in this fluid and contain the foetal DNA repertoire and can therefore be analysed to provide an insight into the genetic integrity of the baby.

An even earlier diagnosis, which can be performed from 12 weeks, is offered by a related technique called CVS - chorionic villus sampling. Here, a needle is introduced into the placenta; since this is of foetal origin, a cell sample of this tissue can enable a high quality genetic analysis.

Both of the aforementioned tests carry a small risk of miscarriage; in most centres this is at or under 1%. Consequently, a newer approach, which involves the extraction of circulating cell-free foetal DNA from the mother's bloodstream and carries no miscarriage risk, is proving popular. This approach uses bioinformatics to work out the relative gene dosages of the foetal DNA. It can also be performed from 10 weeks and hence offers the earliest screening opportunity.