Wolfson Institute of Preventive Medicine

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Reflex DNA Test

The Reflex DNA test involves taking two blood samples in the 11th, 12th or 13th week of pregnancy.

One sample is used straight away to carry out a Combined test while the other sample is retained and only used for a DNA test if this is indicated by the result of the Combined test. 

The Combined test involves:

i) performing an ultrasound scan examination to precisely determine the gestational age of the pregnancy and to measure the nuchal translucency (NT) thickness, a space at the back of the baby’s neck.

ii) taking a sample of your blood to measure the concentration of pregnancy associated plasma protein-A (PAPP-A) and free ß human chorionic gonadotrophin (free ß-hCG).

In pregnancies with Down's syndrome, the PAPP-A level tends to be low and NT measurement and the free ß-hCG level tend to be raised. In pregnancies with Edwards or Patau syndrome the NT measurement tends to be raised while the PAPP-A and free ß-hCG levels tend to be low.

If the risk derived from the Combined test is greater than or equal to 1 in 300 the additional sample taken

for DNA analysis is automatically tested without reporting a result and avoiding the need to recall you for

your DNA test. This automatic testing strategy is called reflex testing. About 1 in 20 women will have their

DNA sample tested in this way.

The DNA test involves:

i) analysing cell free DNA in your blood which contains a small amount of your baby's DNA as well as your own. In pregnancies with Down's, Edwards or Patau syndrome there will be a little bit more DNA from chromosomes 21, 18 or 13 respectively. The test depends on measuring this extra amount.

Currently 5 NHS hospitals provide reflex DNA screening to women booking for their antenatal care; the Royal London, Newham and Whipps Cross Hospitals (Barts Health), Kingston Hospital and Liverpool Women’s Hospital.

For further information about reflex DNA screening click here.

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