Non-Invasive Prenatal Testing (NIPT) is a widely used screening method during pregnancy that provides early information about the genetic health of the developing baby. This test analyzes small fragments of cell-free DNA (cfDNA) that circulate in the mother’s bloodstream, a portion of which originates from the placenta. Spinal Muscular Atrophy (SMA) is a serious, inherited neuromuscular condition that causes the loss of motor neurons and progressive muscle weakness. Because both tests examine fetal genetics, many people ask if standard NIPT screens for SMA risk. The answer requires understanding the distinction between the types of genetic errors each test is designed to detect.
What Standard NIPT Screens For
Standard NIPT is fundamentally a test for chromosomal aneuploidies, which are numerical errors involving entire chromosomes. This screening analyzes cfDNA to detect an abnormal number of copies of specific chromosomes in the fetus. The primary conditions targeted are the common trisomies, where an extra copy of a chromosome is present.
The test focuses on Trisomy 21 (Down syndrome), Trisomy 18 (Edwards syndrome), and Trisomy 13 (Patau syndrome). NIPT also screens for abnormalities involving the sex chromosomes, such as Turner syndrome (Monosomy X) and Klinefelter syndrome (XXY). The technology works by counting the relative amounts of DNA fragments from these specific chromosomes in the maternal blood sample. If the count is higher or lower than expected, it flags a high risk for a corresponding aneuploidy.
NIPT is optimized for detecting these large-scale chromosomal imbalances. These conditions involve an extra or missing copy of an entire chromosome, which is relatively easy to quantify from the small DNA fragments in the blood. Standard NIPT is not designed to detect the much smaller genetic changes that cause single-gene disorders, a category that includes SMA.
The Genetics of Spinal Muscular Atrophy
Spinal Muscular Atrophy is a genetic disorder distinct from the chromosomal aneuploidies screened by standard NIPT. It is classified as an autosomal recessive condition, meaning a child must inherit a mutated copy of the responsible gene from both parents to be affected. SMA is caused by a change in the Survival Motor Neuron 1 (SMN1) gene, located on chromosome 5.
The SMN1 gene provides instructions for making the SMN protein, which is necessary for the health and survival of motor neurons. In approximately 95% of SMA cases, the condition results from a deletion of a section of the SMN1 gene on both copies. This deletion leads to a severe lack of functional SMN protein and the progressive loss of motor function.
A similar gene, SMN2, acts as a disease modifier and can produce a small amount of functional protein. A higher number of SMN2 copies often correlates with a milder form of the disease. The small size of the SMN1 gene deletion means it often falls below the detection threshold for standard NIPT.
Integrating SMA into Prenatal Screening
While SMA is not a target of standard NIPT, it has been integrated into prenatal screening through two distinct approaches. The most common method is Maternal Carrier Screening, which is often offered alongside NIPT but is a separate test. This screening determines if the expectant mother carries a mutation in her SMN1 gene.
If the mother is identified as a carrier, the father is then tested; if both parents are carriers, the risk of the baby having SMA is 25% with each pregnancy. Carrier screening identifies parental risk, while expanded NIPT aims to screen the fetus directly for the presence of the genetic change.
Expanded NIPT Panels
Some specialized laboratories offer expanded NIPT panels that include screening for specific single-gene disorders, including SMA. These panels use advanced sequencing techniques to look for the characteristic SMN1 gene deletion in the fetal cfDNA. This type of test, sometimes called non-invasive prenatal diagnosis (NIPD), analyzes the amount of the SMN1 gene sequence to detect a deletion in the fetus.
The technology used for single-gene NIPD for SMA can employ methods like relative haplotype dosage, which tracks inherited parental gene copies in the fetal DNA. This provides a direct assessment of the baby’s status, rather than just the parents’ carrier status. Patients must confirm with their healthcare provider whether the specific NIPT panel ordered includes this expanded screening, as it is not a universal feature.
Next Steps After Receiving Results
A high-risk result from expanded NIPT for SMA, or confirmation that both parents are carriers, indicates an elevated chance that the baby may be affected. NIPT and carrier screening are screening tools that estimate risk; they do not provide a conclusive diagnosis.
The next step following a high-risk screening result is to pursue diagnostic testing. This typically involves an invasive procedure such as Chorionic Villus Sampling (CVS) or amniocentesis. These procedures collect fetal cells for a direct genetic analysis of the baby’s DNA. The DNA is then analyzed specifically for the SMN1 gene deletion to confirm or rule out SMA.
A high-risk result should be discussed immediately with a genetic counselor or a specialist physician. Genetic counselors are trained to help individuals understand the complex implications of the results, including the specific genetic change detected and the probability of the child being affected. They provide support and guidance through the decision-making process regarding further testing and potential pregnancy outcomes.