A blood test , ultrasound or demographic characteristics (age, race,etc) of a person are used to estimate what are a person’s chances of having an abnormality.In this case a screening test for chromosomal abnormalities is to suspect what are baby’s chances of having that abnormality.
These tests DO NOT mean that there is an abnormality for sure; they only signal further testing should be done.
The Screening can be divided into
First trimester screen
Second trimester screen
What is a first trimester screening for Aneuploidies/Chromosomal Abnormalities?
First Trimester Screen is a new, optional evaluation that combines a maternal blood screening test with an ultrasound evaluation of the fetus(baby in the womb)
It identifies the risk for specific chromosomal abnormalities, including Down syndrome and Trisomy-18 and T13
What is the accuracy of First Trimester Screen ?What does it mean?
The accuracy rate for the combined test to detect the chromosomal abnormalities is upto 95% (depending upon the markers on the scan)with a false positive rate of 3-5%.
This means that:
Approximately 85 out of every 100 babies affected by the abnormalities mentioned above will be identified.
Approximately 5% of normal pregnancies will receive a positive result or an abnormal level and will be labelled as high risk for that condition. This would be a false suspicion confirmed on the diagnostic test.
A positive test means you have a 1/100 to 1/300 chance/risk of having a baby with the said abnormality.
It is worth reiterating that a positive result does not equal to having an abnormality in the baby , but rather serves as a suspicion to prompt further testing.
What are Second Trimester Screening Tests?
They are Triple /Quadruple test as blood test (15-20+6 weeks)
or genetic sonogram as an ultrasound(18-20 weeks).
These screening methods are less accurate than the first trimester combined screen.
What are the Risks and Side Effects to the Mother or Baby?
It includes discomfort of drawing blood and other than this there are no known risks or side effects associated with the screening tests. Ultrasound when used properly (ALARA) is more or less safe for the fetus.
Who should get the Screening Tests done?
All pregnant women must get screening test done at appropriate time irrespective of their age and other factors.