The Double Marker Test is a non-invasive prenatal screening tool performed during the first trimester of pregnancy to estimate the risk of the developing fetus having certain chromosomal abnormalities. This screening relies on a blood sample taken from the expectant mother, which is analyzed for specific biochemical markers. The test provides a statistical risk assessment, indicating the likelihood of a condition, rather than offering a definitive medical diagnosis. A result indicating increased risk suggests the need for further, more conclusive testing.
Components and Timing of the Double Marker Test
The test measures the concentration of two specific substances found in the maternal bloodstream: Pregnancy-Associated Plasma Protein-A (PAPP-A) and Free Beta Human Chorionic Gonadotropin (Free Beta-hCG). PAPP-A is a protein produced by the placenta whose levels increase steadily throughout a healthy pregnancy. Free Beta-hCG is a hormone, a subunit of human chorionic gonadotropin, also produced by placental tissue.
The timing of this test is precise because marker levels change rapidly with gestational age. Healthcare providers recommend performing the blood draw between the 10th and 14th week of gestation, with the ideal window cited as 11 to 13 weeks. Performing the test outside of this timeframe can significantly impact the accuracy of the risk calculation.
The data from the maternal blood test is frequently integrated with information gathered from the Nuchal Translucency (NT) scan. This ultrasound measures the fluid space at the back of the fetus’s neck. Combining the biochemical marker levels with the NT measurement and the mother’s age allows for a more comprehensive combined risk assessment.
Conditions Screened by the Test
The primary purpose of the Double Marker Test is to screen for an increased likelihood of specific chromosomal abnormalities, known as Trisomy, which involve having three copies of a particular chromosome instead of the usual two. The test focuses mainly on two conditions: Trisomy 21 and Trisomy 18.
Trisomy 21, or Down Syndrome, is a genetic condition that causes intellectual disability and distinct physical features.
The test also screens for Trisomy 18, or Edwards Syndrome, which is associated with severe intellectual disability and serious physical abnormalities, often affecting the heart and other organs. Trisomy 18 carries a higher rate of miscarriage or stillbirth. Early identification of risk for both conditions allows parents and medical teams to prepare for these outcomes.
Understanding Risk Assessment and Interpretation
The result is reported as a statistical risk score, not a definitive diagnosis. This score is calculated by an algorithm that integrates the measured levels of PAPP-A and Free Beta-hCG, the gestational age, and the mother’s age. The final output is expressed as a ratio, such as “1 in 500,” meaning that for every 500 pregnancies with this data, one is expected to have the condition.
To standardize the measurement of the biochemical markers, their levels are reported as Multiples of the Median (MoM). The MoM value compares the measured marker level to the median level expected for that gestational age, where 1.0 represents the average value.
Different patterns in the MoM values are associated with increased risk. For Trisomy 21, the typical pattern is a high Free Beta-hCG MoM combined with a low PAPP-A MoM. This combination suggests an atypical placental function.
Conversely, an elevated risk for Trisomy 18 is characterized by MoM values that are low for both Free Beta-hCG and PAPP-A. Healthcare providers set a specific cutoff ratio, such as 1 in 250, to classify a result as “high-risk” or “screen-positive.” Any result exceeding this cutoff, such as 1 in 100, warrants consideration of further investigation. A “low-risk” or “screen-negative” result suggests that the chance of the condition is no higher than the background population risk.
Options Following a High-Risk Result
A high-risk result from the Double Marker Test does not confirm a chromosomal condition but initiates a discussion for next steps. The initial step following a screen-positive result is often Non-Invasive Prenatal Testing (NIPT), which analyzes fetal DNA fragments circulating in the mother’s blood. NIPT is significantly more accurate than the Double Marker Test and provides a refined risk assessment without posing a threat to the pregnancy.
If the NIPT also suggests a high risk, or if the parents wish for a definitive answer immediately, two invasive diagnostic procedures are available. The first option is Chorionic Villus Sampling (CVS), typically performed between the 10th and 13th weeks of gestation. CVS involves taking a small sample of placental tissue, which contains the same genetic material as the fetus, for chromosomal analysis.
The second diagnostic option is Amniocentesis, usually performed from the 15th week of gestation onward. This procedure involves extracting amniotic fluid, which contains fetal cells for genetic testing. While both CVS and Amniocentesis carry a small risk of miscarriage, they provide a conclusive diagnosis necessary to confirm the presence of a chromosomal abnormality.