Introduction
There are more than 200 million pregnancies every year worldwide. The determinants of a healthy pregnancy are preventing, identifying, and addressing risks during pregnancy. Prenatal (before birth) biochemical screening for genetic alterations has become a routine part of the gynecological field in many countries. The double marker test, which is also called maternal serum screening, is done in the first trimester (start of pregnancy till 13 weeks). It includes maternal serum free β-human chorionic gonadotropin (free β-hCG) and pregnancy-associated plasma protein-A (PAPP-A) analysis. The main goal is to identify an increased risk of Down syndrome, Patau syndrome, and Edward syndrome in the offspring.
What Is the Rationale Behind the Double Marker Test?
Various proteins are found in the maternal blood circulation during pregnancy. Differences in levels of some proteins are observed in patients carrying a fetus (an unborn baby) with Down syndrome and other chromosome abnormalities. These differences form the basis for their use in screening protocols. The interpretation of these protein levels (biochemical markers) mandates sound knowledge of the gestational age (the age of pregnancy). It is because certain patterns of biochemical markers are linked to fetal Down syndrome (trisomy 21; extra copy of chromosome 21), Edward Syndrome (trisomy 18; extra copy of chromosome 18), and Patau Syndrome (trisomy 13; extra copy of chromosome 13).
What Are the Biochemical Markers Used in Double Marker Tests?
Various biochemical markers are produced or modified by the placenta (an organ in the uterus during pregnancy).
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Pregnancy-Associated Plasma Protein-A (PAPP-A): PAPP-A is one of the two biomarkers used for the double marker test between ten and 14 weeks of pregnancy. It mediates the glucose and amino acids transport in the placenta. PAPP-A is also expressed in the ovaries, fibroblasts (connective tissue cells), osteoblasts (bone-forming cells), and vascular smooth muscle cells (blood vessel cells). In normal pregnancy, the concentration of PAPP-A increases with gestational age. It rises with a doubling time of three to four days during the first trimester. Then, the level continues to increase throughout pregnancy until childbirth.Decreased levels of PAPP-A are the basis for the first-trimester screening of fetal Down syndrome. PAPP-A levels are also associated with birth weight. A decrease in PAPP-A increases the risk of low birth weight infants. Some other complications of low PAPP-A are preterm delivery, fetal growth restriction, and gestational hypertension (increased maternal blood pressure).
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Maternal Serum Free β-hCG: hCG is a hormone found in blood and urine only during pregnancy. Maternal serum hCG peaks at eight to ten weeks, decreases to reach a plateau at 18 to 20 weeks, and remains constant until delivery. The hormone hCG mediates many placental, uterine, and fetal functions for the initiation and maintenance of pregnancy. These include the development, growth, and differentiation of the endometrium (uterine layer for embryo implantation), maternal immune system suppression (to inhibit immune attack against fetus), gene expression in the uterus, and coordination of signals between the endometrium and uterus. Elevated maternal serum (blood) hCG levels are found in Down’s syndrome pregnancies. Hence, hCG is a part of most screening programs.
The important point is that the results are correlated with an ultrasound called a nuchal translucency (NT) scan (examines the clear tissue at the back of the fetal neck). It is the most crucial ultrasound marker in first-trimester screening for chromosomal abnormalities. The NT measurement must be performed by experienced sonologists between ten and 13 weeks of gestation. NT can be detected in 99 percent of fetuses at the end of the first trimester. Most fetuses have NT ranging from 2 mm (millimeters) in the 11th week 11 to 2.6 mm in the 13th week.
Many Down syndrome fetuses have an increased NT compared to normal fetuses of the same gestational age. Furthermore, an increased NT may also be associated with miscarriage. Screening for biochemical markers and ultrasound can be done in two separate visits (first at 9 to 10 weeks and the second at 12 weeks). If the combined results of the screening point to moderate- or high-risk for abnormalities, genetic counseling is advocated.
What Are the Techniques Used for Double Marker Tests?
The technology for double marker tests provides an automated, precise, and reproductive measurement within a short time after obtaining a blood sample. Serum separation is necessary for Down syndrome screening within four hours after blood withdrawal.
Serum samples for free β-hCG and PAPP-A are stable at 4 degree Celsius (°C) for whole blood for a week. Reliable results are obtained if separated serum samples are stored at 20 °C for up to two days and one day for whole blood. In whole blood, free β-hCG levels increase rapidly at 30 °C.
Presently, all commercial assays are chemiluminescence immune-assay (CLIA) technology. The technology converts a substrate to a reaction product which emits light energy. Maternal serum PAPP-A and free β-hCG can also be measured using a random access immunoassay analyzer using time-resolved amplified cryptate emission technology.
What Are the Advantages and Limitations of the Double Marker Test?
Current studies show that the double marker test can identify about 90 percent of women at risk for Down syndrome and 94 percent of major chromosomal defects such as Patau syndrome, and Edward syndrome. It can also detect approximately 60 percent of other chromosomal defects, such as deletions, partial trisomies, and translocations (one chromosome attaches to another chromosome). First-trimester biochemical screening provides clinicians and patients with the advantage of an earlier diagnosis, higher detection rates for fetal Down syndrome, and detection of other chromosome abnormalities.
It also is a nonspecific marker for some major cardiac (heart) defects and syndromes. Further, it can detect many major structural defects associated with normal chromosomes. However, the double marker test also has certain limitations. One disadvantage is that it does not detect neural tube defects (spinal cord defects). As a result, it requires separate biochemical testing after 15 weeks or a fetal anomaly scan at 18 to 22 weeks (also called a morphology scan).
Another disadvantage is that early screening identifies those abnormal pregnancies that eventually miscarry. About 30 percent of affected fetuses die between 12 weeks of gestation and delivery. Hence, women unnecessarily decide to terminate a pregnancy that miscarries. Also, one must remember that the double marker test only provides the genetic disorder risk but not the diagnosis.
Conclusion
The double marker test is a useful pregnancy screening test to determine healthy fetal development. It is an essential and noninvasive screening test in detecting patients for congenital abnormalities. However, it can also give false-positive results (the result indicates that a condition exists when it does not), leading to invasive (involving incision or surgery) diagnostic procedures, such as amniocentesis (which has a small risk of miscarriage), in risk-free pregnant women. Therefore, the concerned doctors must carefully correlate the test results with the sonographic findings to reduce errors.
