Non invasive prenatal test (NIPT) and first trimester Down's syndrome screening

NIPT is a non-invasive prenatal DNA test developed for the screening of fetal chromosomal abnormalities, such as Down's syndrome(trisomy 21), Edwards syndrome (trisomy 18) and Patau syndrome (trisomy 13). The test is based on proprietary technologies developed at The Chinese University of Hong Kong. During pregnancy, the unborn child releases some of its DNA into the blood circulation of its mother. NIPT is a test that measures the amount of DNA molecules from chromosomes 21, 18, 13 and others in the blood samples of pregnant women. The test can be performed as early as 10 weeks of pregnancy for singleton pregnancy. It is recommended to be done after 12 weeks of pregnancy for twins pregnancy.

NIPT Procedures:

  •  An ultrasound examination to measure the size of the fetus and determinate the pregnancy
  •  Blood taking from pregnant women for assessment

Advantages:

  •  High detection rate (>99%) for common chromosomal abnormalities
  •  Non-invasive ultrasound examination is safe to the fetus
  •  Screening result not affected by maternal age
  •  Early detection, the test can be performed as early as 10 weeks of pregnancy. The turnaround time for test is 5-8 working days
  •  If the fetus is confirmed with Down's syndrome, there is still enough time to consider management options
  •  If termination of pregnancy is chosen for an abnormal fetus, it carries less risks to the women when performed in early pregnancy

“Positive” results:
Do not 100% confirm that the fetus is affected with chromosomal abnormality. There is a chance of a false positive result. So, positive test results should be confirmed by Amniocentesis or Chorionic Villus Sampling.

“Negative” results:
The chance of fetus affected with Down ‘s syndrome, trisomy 18, trisomy 13, sex chromosomal abnormalities and common microdeletion syndrome is very low. However, it is also important to point out that safe T21 remains a screening test. It cannot completely exclude the possibility of having a fetus with these chromosome abnormalities. So Invasive testing (eg. Amniocentesis or Chorionic Villus Sampling) may sometimes be necessary to confirm the diagnosis. References:http://www.xcelom.com/

First trimester combined Down's syndrome screening

First trimester combined Down's syndrome screening is a screening method performed at 11-14 weeks of pregnancy. The following parameters are obtained to calculate the risks of having a Down’s fetus for each individual woman:

  •  The woman’s age
  •  Her serum level of pregnancy-associated plasma protein-A (PAPP-A)
  •  Her serum level of free beta human chorionic gonadotropin (free-beta hCG)
  •  Thickness of the fetus neck-fold (nuchal translucency thickness)

Advantages of first trimester Down's syndrome screening :

  •  High detection rate (~90%) for common chromosomal abnormalities
  •  Non-invasive ultrasound examination is safe to the fetus
  •  Suitable for women of any age
  •  It can be performed in early pregnancy and hence there is plenty of time to further confirm the results be positive
  •  If the fetus is confirmed with Down's syndrome, there is still enough time to consider management options
  •  If termination of pregnancy is chosen for an abnormal fetus, it carries less risks to the women when performed in early pregnancy

“Positive” OR “Increased risk” results:
Do not mean that the fetus is abnormal. It only means that further invasive tests, such as amniocentesis or CVS, need to be considered to confirm whether the fetus is normal or has Down's syndrome.

“Negative” OR “Low risk” results:
Only indicate that the risk that the fetus has Down's syndrome as compared to that of pregnant women aged 35 is low. However, it does not completely eliminate the possibility that the fetus may have Down's syndrome or other chromosomal abnormalities.