Non-Invasive Prenatal Testing (NIPT) is a screening tool that offers expectant parents an early assessment of the risk for certain genetic conditions in the developing fetus. The test is performed using a simple blood draw from the mother and poses no physical risk to the pregnancy. NIPT analyzes fragments of genetic material to estimate the likelihood of chromosomal abnormalities, such as those involving extra or missing chromosome copies. This information guides further decisions and diagnostic procedures in prenatal care.
Optimal Timing for NIPT
NIPT can typically be performed starting at 10 weeks of gestation. This timing is dictated by the requirement for a sufficient quantity of cell-free fetal DNA (cfDNA) to be circulating in the mother’s blood.
The necessary concentration of fetal DNA is known as the fetal fraction, and most labs require it to be at least 4% for a reliable result. Before 10 weeks, the fetal fraction is often too low, which can lead to an “inconclusive” result. This means the test could not provide a clear risk assessment and typically requires a repeat blood draw.
While NIPT can be performed anytime after the 10-week mark, many healthcare providers recommend completing it between 10 and 14 weeks. Testing within this early window allows for the earliest possible risk assessment and provides more time for follow-up diagnostic procedures if a high-risk result is returned. Performing the test later does not impact accuracy but may limit the time available for further decision-making.
The Mechanism of Cell-Free DNA
The foundation of NIPT relies on the presence of cell-free DNA (cfDNA) in the pregnant person’s blood. These are small fragments of DNA released when cells naturally break down and die. During pregnancy, the mother’s bloodstream contains her own cfDNA mixed with cfDNA originating from the placenta, which is genetically identical to the fetus.
The test isolates and analyzes these DNA fragments to determine the fetal fraction and count the number of copies for specific chromosomes. If the test detects a disproportionate amount of fragments from a particular chromosome, it suggests a higher likelihood of an extra or missing chromosome copy in the fetus. Since the test analyzes placental DNA, and not the fetus directly, it is considered a non-invasive screening method.
Conditions Screened By NIPT
NIPT is specifically designed to screen for the most common chromosomal aneuploidies, which involve an atypical number of chromosomes. The primary focus is on the three most frequent trisomies, where an individual has three copies of a chromosome instead of the usual two.
These conditions include Trisomy 21 (Down syndrome), which is the most frequent trisomy at birth. The test also screens for Trisomy 18 (Edwards syndrome) and Trisomy 13 (Patau syndrome).
Many NIPT panels also include screening for sex chromosome aneuploidies, which involve extra or missing copies of the X or Y chromosomes. Examples include Turner syndrome (Monosomy X) and Klinefelter syndrome (XXY). NIPT is a screening method only and cannot detect all genetic conditions or structural birth defects.
Interpreting and Following Up on Results
The results of NIPT are typically reported as either “low risk” or “high risk” for the conditions screened. A low-risk result means it is highly unlikely the fetus has the condition. Conversely, a high-risk result indicates an increased probability that the condition is present, but it is not a definitive diagnosis.
The reliability of a high-risk result is measured by its Positive Predictive Value (PPV), which is the chance that a positive test is truly positive. The PPV varies based on the condition being screened for and the mother’s individual risk factors, such as age. For example, the PPV for Down syndrome is generally higher than for rarer conditions.
If a high-risk result is returned, the next recommended step is genetic counseling to discuss the implications and options for diagnostic testing. Diagnostic procedures, such as chorionic villus sampling (CVS) or amniocentesis, are invasive but analyze fetal cells directly to provide a definitive answer. The NIPT result may also come back as “inconclusive” or “no call,” typically due to a low fetal fraction, which necessitates a retest or a move to other screening options.