Prenatal screening tests for chromosomal abnormalities
Prenatal screening tests for chromosomal abnormalities
sara sanjarian,1,*
1. Academic Center for Education, Culture and Research-Fars Branch Institute for Human Genetics Research
Introduction: The health of the fetus is the main concern of all parents, and chromosomal abnormality in the fetus is one of the main causes of stillbirths and the birth of infants with abnormalities. Since it is possible for every pregnant woman to give birth to a child with birth defects, performing screening tests before birth makes it possible to be aware of chromosomal health. Over the past years, different methods and procedures have been used to identify the chromosomal status of the fetus before birth. These procedures are divided into two groups: invasive and non-invasive. Amniocentesis and chorionic villus sampling (CVS) are invasive procedures, while non-invasive methods include ultrasound, first-trimester screening, second-trimester screening, and non-invasive prenatal testing (NIPT). In ultrasound, performed between weeks 11 and 14 of pregnancy, factors such as nuchal translucency (NT), crown-rump length (CRL), and the presence or absence of a nasal septum are of paramount importance. Typically, the screening of the first trimester of pregnancy is performed between weeks 11 and 13 of pregnancy. The free beta-human chorionic gonadotropin (β-hCG) and pregnancy-associated plasma protein A (PAPP-A), which are produced during pregnancy, are measured in the blood. Finally, through NT ultrasound information and other important factors such as mother’s age, diabetes, other children's history of chromosomal abnormalities, twin pregnancy, and smoking, a numerical risk is obtained based on which, we can decide for the next step. In the screening of the second trimester, which is best performed between weeks 15 and 18 of pregnancy, the levels of four substances of alpha-fetoprotein (AFP), unconjugated oestriol (UE3), inhibin-A, and β-HCG are measured in the blood, and combined with the factors mentioned in the first trimester, the risk of chromosomal abnormalities is obtained.
NIPT, also known as cell-free DNA, can be performed from 10 weeks of pregnancy. In this test, which is performed on the mother’s blood, parts of the fetal DNA that entered the mother’s blood circulation from the placenta are examined. NIPT test has higher sensitivity and specificity in detecting trisomies compared to other pregnancy screening tests. Non-invasiveness and quick response are other advantages of this test, leading to the growing use of this test around the world. However, it also has some drawbacks, causing NIPT not to be still considered as a diagnostic test, and if its results turn out to be positive, administrating such diagnostic tests as aminosynthesis and CVS will be necessary to confirm it. One drawback of NIPT is that it is possible that the DNA circulating in the mother’s blood develops from the placenta and not from the fetus, and the chromosomal abnormality is related to the placenta that does not affect the fetus. Further, it is also likely to get false positive or negative results where the mother has a tumor, has a low fetal fraction in the blood, is pregnant with multiples (more than twins), and is overweight.
Methods: this article is a literature review
Results: To get information regarding the chromosomal status of the fetus, non-invasive tests such as first-trimester screening, second-trimester screening, or NIPT are used. Among these, NIPT has the highest sensitivity and specificity, but it is not considered a diagnostic test due to due to its limitations. Amniocentesis and CVS are accurate prenatal diagnostic tests, which are invasive and carry the risk of miscarriage.
Conclusion: Performing aminosynthesis and CVS tests requires sufficient experience and high expertise in this field. The growing use of NIPT in many countries has reduced the need to use invasive tests (e.g., aminosynthesis and CVS), leading to a decrease in the number of experienced specialists performing diagnostic tests. Therefore, researchers are trying to overcome the disadvantages of the NIPT test by expanding the science and technology so that it can be applied as a non-invasive diagnostic test in the future.