A new blood test for Down syndrome has recently become available. The laboratory is now able to extract free fetal DNA from the pregnant woman’s blood, to test for extra fragments of chromosome 21.
If extra fragments of chromosome 21 are detected it can signify the presence of Trisomy 21 (Down syndrome) in the fetus. This test also detects extra fragments of chromosome 18 and 13 and can also detect some sex chromosome abnormalities. It will tell you the sex of the baby.
This test is reported to have a detection rate of 99% for Down syndrome. It is not 100% diagnostic so it is really a screening test.
It has a better detection rate than any other screening test we have, such as nuchal translucency screening, combined nuchal and serum screening and the triple test.
It can be done from 10 weeks gestation onwards.
It is suitable for twin pregnancies but there is not enough information to know whether it is suitable for triplets.
If the result is "high risk" for Down syndrome, Trisomy 18 or Trisomy 13 then a confirmatory test like a chorionic villus sampling or amniocentesis is necessary.
We prefer amniocentesis because there is a lower chance of a confusing result from confined placental mosaicism.
Cell free fetal DNA has become extremely popular either
as a follow-up test after nuchal translucency screening shows a high risk result
An ultrasound examination at 12-14 weeks gestation is still recommended to confirm the gestational age, detect the presence of twins, and to look for major structural abnormalities which may be obvious early.
An ultrasound examination at 19-20 weeks gestation is still recommended to detect structural abnormalities which may become obvious, and to determine the position of the placenta and the volume of amniotic fluid around the baby.
If you would like to discuss this option for your pregnancy, please make an appointment to see one of our specialist obstetricians or genetic counselors.
We specialise in prenatal diagnosis and ultrasound.
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