The triple screening test, also known as triple screen, or triple test, is a maternal blood screening that is performed during the first trimester of pregnancy. The test screens for three substances, that when present in the blood may indicate the risk for birth defects.
Who should undergo the test?
The triple screen is offered to all pregnant women, regardless of risk factors. This is a simple, non-invasive blood test with no risk to the mother or fetus.
How is the test performed?
The test is usually performed between weeks 15-22 of pregnancy. Deviations from this timeframe may affect the test’s accuracy.
The test involves a blood test in which one sample of blood is drawn from the mother. The blood sample is then sent to a laboratory for analysis.
The test measures three substances:
- Alpha-fetoprotein (AFP) – a protein produced by the fetus.
- hCG –human chorionic gonadotropin – a hormone produced in the placenta.
- Estriol – an estrogen (hormone) produced during pregnancy.
Each of these three tests contributes to the results and can indicate a risk for different diseases.
Today, a fourth test is often added – the hormone inhibin A. In these cases, it is known as a quad screen or quadruple marker test. The quad screen is known for a higher accuracy rate in detecting Down syndrome.
What diseases can the test detect?
The triple test does not diagnose diseases but only indicates a risk for certain disorders and raises the need for further testing.
The test may suggest that the fetus is at risk of the following diseases:
- Chromosome abnormalities, such as Down syndrome (trisomy 21), trisomy 18, and trisomy 13.
- Neural tube defects- such as spina bifida
- Intestinal defects such as Gastroschisis or fetal omphalocele.
Twin or triplet pregnancies can cause irregular triple screen results. In a multiple pregnancy, the AFP levels are measured in MOMs – Multiple of Medians, test results are multiplied by the median result. Usually, an MOM value of over 2.5 requires further testing.
Results
The results are statistical, and not quantitative. They indicate a risk for birth defects, in combination with maternal age. The different components of the screen can indicate different diseases.
- Low levels of AFP combined with high levels of hCG are linked to a high risk of Down syndrome.
- Low levels od AFP with low levels of hCG may indicate a risk for trisomy 18
- High levels of AFP may indicate a risk for neural tube defects.
Further tests
After abnormal test results, the recommendation is to complete prenatal tests, especially Amniocentesis and anatomy scans.
An ultrasound exam is offered to check the fetus’ body and detect possible anatomical defects. Certain defects – such as heart and intestinal defects, are typical of Down syndrome. Nervous system defects may also be detected in an ultrasound.
Amniocentesis is an invasive prenatal test that can detect chromosome abnormalities and genetic disorders.
