The “Stork Test” is a widely recognized term for Non-Invasive Prenatal Screening (NIPS) or Testing (NIPT). This advanced method assesses the risk of a fetus having certain genetic conditions by analyzing tiny fragments of the baby’s genetic material circulating in the pregnant person’s bloodstream. It is a screening tool, meaning it estimates the probability or risk of a condition being present, but it does not provide a definitive diagnosis.
How the Stork Test Works (The Role of Cell-Free DNA)
The scientific basis of this screening relies on the presence of cell-free DNA (cfDNA) in the maternal circulation. These are short fragments of DNA that are not contained within cells, having been released into the blood when cells naturally break down. During pregnancy, the pregnant person’s bloodstream contains a mixture of their own cfDNA and cfDNA originating primarily from the placenta, which shares the baby’s genetic code.
The cfDNA from the placenta, known as the fetal fraction, is the target for analysis. A laboratory extracts this cfDNA from the blood sample and uses high-throughput sequencing technology to count and analyze the fragments.
If the fetus has an extra copy of a chromosome (a trisomy), the test detects a greater-than-expected count of DNA fragments corresponding to that chromosome. This counting allows the test to identify an increased or decreased likelihood of a chromosomal difference. For reliable results, the fetal fraction must typically be above a certain threshold, often around 4%, which usually occurs by the tenth week of pregnancy.
Genetic Conditions Screened
The Stork Test, like other NIPTs, primarily focuses on screening for the most common chromosomal aneuploidies. These conditions result from having an extra copy of a specific chromosome. The main conditions screened for are Trisomy 21 (Down syndrome), Trisomy 18 (Edwards syndrome), and Trisomy 13 (Patau syndrome).
Trisomy 21 (Down syndrome) is the most frequently detected trisomy at birth, associated with intellectual disability. Trisomy 18 and Trisomy 13 are generally more severe, often resulting in a high rate of miscarriage or a very short life expectancy after birth.
The screening also commonly includes an analysis of the sex chromosomes (X and Y) to look for sex chromosome aneuploidies, such as Turner syndrome or Klinefelter syndrome, where there is a missing or extra sex chromosome. Many NIPT panels also offer the option to determine the sex of the fetus. Some advanced tests can additionally screen for smaller missing or duplicated sections of chromosomes, known as microdeletion or microduplication syndromes, though this is not standard across all platforms.
When and How the Test Is Administered
The test is considered non-invasive because it only requires a standard blood draw from the pregnant person’s arm, posing no physical risk to the fetus. The earliest the test can be performed is typically around 10 weeks of gestation. This timing is important because it is generally when the concentration of fetal cfDNA in the mother’s blood, the fetal fraction, is sufficiently high to allow for accurate analysis.
The blood sample is collected by a healthcare provider and sent to a specialized laboratory for processing and genetic sequencing. The turnaround time for results can vary based on the laboratory, but generally falls within a range of 7 to 14 calendar days. Occasionally, a repeat blood draw may be requested if the initial sample did not contain a sufficient amount of fetal DNA, which can happen if the test is performed too early.
Understanding the Test Results
The results of the Stork Test are typically reported as either “Low Risk” (negative) or “High Risk” (positive) for each of the conditions screened. A Low Risk result indicates a decreased likelihood that the fetus has the condition. Conversely, a High Risk result suggests an increased probability of the condition being present.
Even a High Risk result does not confirm a diagnosis, and the result may be a false positive, meaning the test indicates a problem that is not actually present. NIPT is highly accurate for Trisomy 21, with a detection rate around 99%, but the accuracy is slightly lower for Trisomy 18 and 13.
If a High Risk result is returned, the next step involves genetic counseling to discuss the implications and options for confirmation. To definitively confirm or rule out a diagnosis, a follow-up invasive diagnostic procedure, such as chorionic villus sampling (CVS) or amniocentesis, is necessary. These diagnostic tests analyze fetal cells directly and provide a conclusive answer, but they carry a small risk of complication, including miscarriage.