These tests are used to screen mothers for possible foetal abnormalities and assess the risk of them giving birth to a defective baby. These tests are offered to all pregnant women. However, these screening tests cannot be said to be always accurate, they may miss abnormalities and sometimes indicate abnormalities when they are not present. The Screening tests are classified as First trimester screening, and Second trimester screening.
First trimester screening
Tests offered in the first trimester include maternal serum screening and an ultrasound scan.
Maternal serum screening are blood tests to measure two types of protein formed by the placenta during early stage of pregnancy. The proteins formed are pregnancy associated plasma protein and human chorionic gonadotrophin.
Abnormal levels of these protein in the blood of pregnant women would indicate possibility of chromosomal abnormalities.
Ultrasound scan measures the thickness of the skin at the back of the neck of the foetus, known as nuchal transculency or nuchal fold thickness. An increased nuchal thickness indicates possible chromosomal abnormalities and heart defects.
Maternal blood test is done around the 10th week of pregnancy and ultrasound scan is done during the 12th week of pregnancy. Results of maternal blood test and ultrasound scan together can determine the risk level of the baby getting birth defects like Down syndrome, Trisomy 13 or 18 and neural tube defects. A positive result shows an increased risk for birth defects, while a negative result shows that the chances for a birth defect is low. If you have an increased risk, you will be referred to a genetic counsellor, to get to know in detail about the risk level for a birth defect, and to decide based on the severity of the problem, either to terminate or continue with the pregnancy.
Second trimester screening
The time period for this screening is between 15 to 20 weeks. These include blood tests to measure certain markers like alpha-feto protein and estriol hormone produced by the foetus, human chorionic gonadotrophin hormone and inhibinA hormone produced by the placenta, and ultrasound scan.
Alpha-feto protein level is measured in all women, even in those who have had first trimester screening. Abnormal level would indicate that the baby is at a risk of having neural tube defects. Measuring the level of first three markers is known as triple screening and if inhibinA is also measured, it is known as Quad screening. Triple or Quad screening is done between 15 and 20 weeks of pregnancy. This test calculates the risk of having a baby with Down syndrome, Trisomy 13 or 18 or other chromosomal abnormalities. Results of blood test and scan together can confirm the risk of chromosomal abnormalities in the baby.
Carrier screening test
Carrier screening for genetic disorders are done to find if the parents, either one or both of them of the unborn baby, are carriers of a defective gene for any of the inherited disorders. If a particular genetic disorder runs in your family history, then you should undergo the carrier screening test. This testing is done before, or during pregnancy to assess the risk of these disorders being passed on to the baby.