Non-Invasive Prenatal Testing (NIPT) is a screening tool that analyzes a sample of the mother’s blood to assess the risk for certain chromosomal conditions in the developing fetus. Because it only requires a simple blood draw, it poses no physical risk to the pregnancy. NIPT provides an early and highly accurate risk assessment before more invasive testing methods are considered.
Determining the Earliest Testing Window
The earliest time an NIPT can be performed is typically at the start of the tenth week of pregnancy. Most guidelines recommend waiting until at least 10 weeks of gestation to ensure a sufficient quantity of fetal material is present in the maternal bloodstream. Performing the test earlier is not recommended and increases the chance of an inconclusive result.
The optimal window for the NIPT blood draw is generally considered to be between 10 and 22 weeks of gestation. Early results provide parents with more time to consider and arrange any potential follow-up diagnostic testing or counseling services.
Fetal Fraction and Timing Limitations
The timing restriction for NIPT is based on the “fetal fraction.” The test analyzes cell-free DNA (cfDNA), which are fragments of DNA circulating in the mother’s blood. This cfDNA comes from both the mother and the placenta, and the placental DNA serves as a proxy for fetal DNA.
The fetal fraction (FF) is the percentage of total cfDNA originating from the placenta. For the test to accurately detect chromosomal differences, the FF must reach a minimum threshold, typically cited as 4% by NIPT providers. Before 10 weeks of gestation, the fetal fraction is often too low to meet this minimum, making the test less reliable. A low fetal fraction results in a “no-call” or failed result, meaning the laboratory cannot provide a risk assessment and a repeat blood draw is necessary.
What NIPT Screens For
NIPT screens for the most common chromosomal aneuploidies, which result from an incorrect number of chromosomes. The test primarily focuses on the three most frequent trisomies: Trisomy 21 (Down syndrome), Trisomy 18 (Edwards syndrome), and Trisomy 13 (Patau syndrome). NIPT also screens for sex chromosome aneuploidies, which involve differences in the number of X or Y chromosomes, such as Monosomy X (Turner syndrome) or Klinefelter syndrome (XXY).
Microdeletions
Some expanded NIPT panels may screen for microdeletions, which are conditions caused by a small missing segment of a chromosome. Examples include 22q11.2 deletion syndrome and Prader-Willi/Angelman syndromes. NIPT’s accuracy for microdeletions is lower compared to its accuracy for the main trisomies, and these are often offered as optional screens.
Understanding Your NIPT Results
NIPT is a screening test, not a diagnostic test, meaning it provides a risk assessment, not a definitive diagnosis. A low-risk result suggests it is highly unlikely the fetus has the screened-for condition. A high-risk result indicates an increased chance of the condition, but it does not confirm a diagnosis.
Because false positives can occur, a high-risk result must always be confirmed with a diagnostic procedure. Confirmatory diagnostic testing, such as chorionic villus sampling (CVS) or amniocentesis, is necessary to establish a certain diagnosis. These follow-up tests analyze fetal cells directly and provide a definitive answer, but they are invasive procedures that carry a small risk of complication. Consultation with a healthcare provider or a genetic counselor is the next step after a high-risk NIPT result.